Optical glass composition, preform and optical element

ABSTRACT

An optical glass composition contains, in % by mole, 0% or more and 25.0% or less of SiO 2 , 20.0% or more and 40.0% or less of B 2 O 3 , 0% or more and 5.0% or less of Li 2 O, 3.0% or more and 15.0% or less of ZnO, 0% or more and 10.0% or less of ZrO 2 , 2.0% or more and 7.0% or less of Ta 2 O 5 , 6.0% or more and 25.0% or less of La 2 O 3 , 5.0% or more and 22.0% or less of Gd 2 O 3 , 66.5% or less of La 2 O 3 +Gd 2 O 3 +B 2 O 3  and 26.0% or more of La 2 O 3 +Gd 2 O 3 , and has nd of 1.83 or higher and 1.86 or lower, vd of 43 or higher and 46 or lower and a liquidus temperature of 1300° C. or lower, and a preform and an optical element are formed from the optical glass composition.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on application Nos. 2008-284326, 2008-284327,2008-284328, 2008-284329 and 2008-284330 filed in Japan on Nov. 5, 2008,the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical glass composition, a preformand an optical element. In particular, the present invention relates to:an optical glass composition suitable as the material of an opticalelement such as a lens element contained in a shooting lens system of adigital still camera or a digital video camera (simply referred to as adigital camera, hereinafter); a preform used in producing this opticalelement by press molding; and this optical element.

2. Description of the Background Art

In recent years, in digital cameras, various types ranging from commontypes to high-class types have been proposed in accordance with theconsumers' needs. Among such digital cameras, in high-class type digitalcameras particularly, high performances such as both of wide angle andhigh magnification are strongly desired as well as thickness reductionfor improving portability.

In order to achieve thickness reduction in a digital camera, reductionis indispensable in the thickness of the shooting lens system whichoccupies a relatively large volume. For the purpose of thicknessreduction in a shooting lens system, reduction in the number of lenselements is effective. Nevertheless, in recent years, increasingly highoptical performance is required in shooting lens systems. Thus,reduction in the number of lens elements is reaching a limit forincreasing magnification particularly, and hence further reduction isunexpectable. Accordingly, for the purpose of thickness reduction in ashooting lens system, thickness reduction becomes necessary in theindividual lens elements contained in the shooting lens system.

For the purpose of thickness reduction in a lens element, it iseffective to increase the refractive index of the glass material thatforms the lens element. For simultaneously achieving the above-mentionedhigh performances such as both of wide angle and high magnification, itis generally preferable that the dispersion of the lens element isadjusted to relatively lower. As examples of such a glass materialhaving a high refractive index and showing a relatively lowerdispersion, optical glass is described in Japanese Laid-Open PatentPublication No. 2003-267748.

In a shooting lens system, it is expected that high performances such asboth of wide angle and high magnification in addition to thicknessreduction can be realized when a glass material having a high refractiveindex and showing a relatively lower dispersion is used, for example,for a most object side-negative lens element in a lens unit havingnegative optical power and also for a positive lens element in a lensunit located on the image side of the lens unit having negative opticalpower. Also, in an imaging lens system, aberration compensation isnecessary. Then, in general, the aberration is compensated by variouslycombining the optical indices such as the refractive indices (nd) andthe dispersions (vd: Abbe numbers) of the lens elements and the shapesof the lens elements. Thus, in order to achieve successful aberrationcompensation as well as both of thickness reduction and highperformances in a shooting lens system, it is desired that for a lenselement, a glass material having an Abbe number falling within aprescribed range relative to a high refractive index falling within aprescribed range is selected from glass materials having a highrefractive index and showing a relatively lower dispersion.Simultaneously, it is desired that the optical power, the shape and thelike of the lens element formed from such a glass material are adjustedto suitable ones, and that the lens element is located on a suitableposition.

Japanese Laid-Open Patent Publication No. 2003-267748 discloses opticalglass having optical indices such as a refractive index of 1.88 to 1.90and an Abbe number of 35 to 50. In this optical glass, the refractiveindex and the Abbe number are defined by a prescribed condition. Theoptical glass has such a high refractive index. Nevertheless, thedispersion of the optical glass falls within wide range from lower tohigher. In order to achieve successful aberration compensation as wellas both of thickness reduction and high performances in a shooting lenssystem, such a lens element is needed as a lens element formed from, forexample, a glass material having an Abbe number of approximately 45relative to a high refractive index of approximately 1.85. Nevertheless,Japanese Laid-Open Patent Publication No. 2003-267748 does notspecifically describe a glass material having the above-mentionedspecific combination of the refractive index with the Abbe number, andshowing excellent melting property, processability (droplet property),crystallinity and the like.

SUMMARY OF THE INVENTION

An object of the present invention is to provide: a high refractiveindex-lower to middle dispersion type optical glass composition that hasa refractive index (nd) to the d-line falling within a high refractiveindex range of approximately 1.85 and an Abbe number (vd) to the d-linefalling within a range of approximately 45, and yet shows excellentmelting property, processability (droplet property), crystallinity andthe like; and a preform and an optical element formed from the opticalglass composition.

(I) The novel concepts disclosed herein were achieved in order to solvethe foregoing problems in the conventional art, and herein is disclosed:

an optical glass composition comprising, in % by mole,

0% or more and 25.0% or less of SiO₂,

20.0% or more and 40.0% or less of B₂O₃,

0% or more and 5.0% or less of Li₂O,

3.0% or more and 15.0% or less of ZnO,

0% or more and 10.0% or less of ZrO₂,

2.0% or more and 7.0% or less of Ta₂O₅,

6.0% or more and 25.0% or less of La₂O₃,

5.0% or more and 22.0% or less of Gd₂O₃,

66.5% or less of La₂O₃+Gd₂O₃+B₂O₃ and

26.0% or more of La₂O₃+Gd₂O₃, and having

a refractive index (nd) to the d-line of 1.83 or higher and 1.86 orlower, an Abbe number (vd) to the d-line of 43 or higher and 46 orlower, and a liquidus temperature of 1300° C. or lower.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

a preform comprising just above-mentioned optical glass composition,that is softened by heating so as to be used at least for press molding.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

an optical element comprising just above-mentioned optical glasscomposition.

(II) The novel concepts disclosed herein were achieved in order to solvethe foregoing problems in the conventional art, and herein is disclosed:

an optical glass composition comprising, in % by mole,

5.0% or more and 25.0% or less of SiO₂,

25.0% or more and 40.0% or less of B₂O₃,

10.0% or more and 15.0% or less of ZnO,

0% or more and 5.0% or less of ZrO₂,

10.0% or more and 25.0% or less of La₂O₃,

5.0% or more and 20.0% or less of Gd₂O₃ and

0% or more and 5.0% or less of Ta₂O₅, wherein

La₂O₃/Gd₂O₃ is, in molar ratio, more than 0 and less than 3.5, wherein

the composition substantially contains no Li₂O, and wherein

the composition has a refractive index (nd) to the d-line of 1.83 orhigher and 1.87 or lower, an Abbe number (vd) to the d-line of 43 orhigher and 47 or lower, and a liquidus temperature of 1300° C. or lower.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

a preform comprising just above-mentioned optical glass composition,that is softened by heating so as to be used at least for press molding.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

an optical element comprising just above-mentioned optical glasscomposition.

(III) The novel concepts disclosed herein were achieved in order tosolve the foregoing problems in the conventional art, and herein isdisclosed:

an optical glass composition comprising, in % by mole,

5.0% or more and 25.0% or less of SiO₂,

20.0% or more and 40.0% or less of B₂O₃,

0% or more and 15.0% or less of ZnO,

0% or more and 5.0% or less of ZrO₂,

10.0% or more and 25.0% or less of La₂O₃,

0% or more and 5.0% or less of Ta₂O₅ and

5.0% or more and 20.0% or less of Gd₂O₃, wherein

SiO₂/B₂O₃ is, in molar ratio, 0.25 or more and 0.90 or less, wherein

the composition substantially contains no Li₂O, and wherein

the composition has a refractive index (nd) to the d-line of 1.83 orhigher and 1.87 or lower, an Abbe number (vd) to the d-line of 43 orhigher and 47 or lower, and a liquidus temperature of 1300° C. or lower.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

a preform comprising just above-mentioned optical glass composition,that is softened by heating so as to be used at least for press molding.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

an optical element comprising just above-mentioned optical glasscomposition.

(IV) The novel concepts disclosed herein were achieved in order to solvethe foregoing problems in the conventional art, and herein is disclosed:

an optical glass composition comprising, in % by mole,

0% or more and 10.0% or less of SiO₂,

30.0% or more and 45.0% or less of B₂O₃,

0% or more and 5.0% or less of Li₂O,

0% or more and 12.0% or less of ZnO,

0% or more and 10.0% or less of ZrO₂,

10.0% or more and 20.0% or less of La₂O₃,

3.0% or more and 10.0% or less of Ta₂O₅,

11.0% or more and 20.0% or less of Ta₂O₅+ZrO₂ and

5.0% or more and 20.0% or less of Gd₂O₃, and having

a refractive index (nd) to the d-line of 1.83 or higher and 1.86 orlower, an Abbe number (vd) to the d-line of 43 or higher and 45 orlower, and a liquidus temperature of 1200° C. or lower.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

a preform comprising just above-mentioned optical glass composition,that is softened by heating so as to be used at least for press molding.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

an optical element comprising just above-mentioned optical glasscomposition.

(V) The novel concepts disclosed herein were achieved in order to solvethe foregoing problems in the conventional art, and herein is disclosed:

an optical glass composition comprising, in % by mole,

0% or more and 10.0% or less of SiO₂,

35.0% or more and 45.0% or less of B₂O₃,

0% or more and 5.0% or less of Li₂O,

0% or more and 12.0% or less of ZnO,

0% or more and 10.0% or less of ZrO₂,

10.0% or more and 20.0% or less of La₂O₃,

3.0% or more and 10.0% or less of Ta₂O₅,

10.0% or more and 22.0% or less of Ta₂O₅+ZnO and

5.0% or more and 20.0% or less of Gd₂O₃, and having

a refractive index (nd) to the d-line of 1.83 or higher and 1.86 orlower, an Abbe number (vd) to the d-line of 43 or higher and 46 orlower, and a liquidus temperature of 1200° C. or lower.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

a preform comprising just above-mentioned optical glass composition,that is softened by heating so as to be used at least for press molding.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

an optical element comprising just above-mentioned optical glasscomposition.

(I) The present invention realizes

a high refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.86 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 46 or lower, and yet has a liquidus temperature of 1300° C.or lower.

The present invention further realizes

a preform used in producing an optical element by press molding from ahigh refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.86 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 46 or lower, and yet has a liquidus temperature of 1300° C.or lower.

The present invention yet further realizes

an optical element formed from a high refractive index-lower to middledispersion type optical glass composition that has a refractive index(nd) to the d-line falling within a high refractive index range of 1.83or higher and 1.86 or lower and an Abbe number (vd) to the d-linefalling within a range of 43 or higher and 46 or lower, and yet has aliquidus temperature of 1300° C. or lower.

(II) The present invention realizes

a high refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.87 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 47 or lower, and yet has a liquidus temperature of 1300° C.or lower.

The present invention further realizes

a preform used in producing an optical element by press molding from ahigh refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.87 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 47 or lower, and yet has a liquidus temperature of 1300° C.or lower.

The present invention yet further realizes

an optical element formed from a high refractive index-lower to middledispersion type optical glass composition that has a refractive index(nd) to the d-line falling within a high refractive index range of 1.83or higher and 1.87 or lower and an Abbe number (vd) to the d-linefalling within a range of 43 or higher and 47 or lower, and yet has aliquidus temperature of 1300° C. or lower.

(III) The present invention realizes

a high refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.87 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 47 or lower, and yet has a liquidus temperature of 1300° C.or lower.

The present invention further realizes

a preform used in producing an optical element by press molding from ahigh refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.87 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 47 or lower, and yet has a liquidus temperature of 1300° C.or lower.

The present invention yet further realizes

an optical element formed from a high refractive index-lower to middledispersion type optical glass composition that has a refractive index(nd) to the d-line falling within a high refractive index range of 1.83or higher and 1.87 or lower and an Abbe number (vd) to the d-linefalling within a range of 43 or higher and 47 or lower, and yet has aliquidus temperature of 1300° C. or lower.

(IV) The present invention realizes

a high refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.86 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 45 or lower, and yet has a liquidus temperature of 1200° C.or lower.

The present invention further realizes

a preform used in producing an optical element by press molding from ahigh refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.86 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 45 or lower, and yet has a liquidus temperature of 1200° C.or lower.

The present invention yet further realizes

an optical element formed from a high refractive index-lower to middledispersion type optical glass composition that has a refractive index(nd) to the d-line falling within a high refractive index range of 1.83or higher and 1.86 or lower and an Abbe number (vd) to the d-linefalling within a range of 43 or higher and 45 or lower, and yet has aliquidus temperature of 1200° C. or lower.

(V) The present invention realizes

a high refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.86 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 46 or lower, and yet has a liquidus temperature of 1200° C.or lower.

The present invention further realizes

a preform used in producing an optical element by press molding from ahigh refractive index-lower to middle dispersion type optical glasscomposition that has a refractive index (nd) to the d-line fallingwithin a high refractive index range of 1.83 or higher and 1.86 or lowerand an Abbe number (vd) to the d-line falling within a range of 43 orhigher and 46 or lower, and yet has a liquidus temperature of 1200° C.or lower.

The present invention yet further realizes

an optical element formed from a high refractive index-lower to middledispersion type optical glass composition that has a refractive index(nd) to the d-line falling within a high refractive index range of 1.83or higher and 1.86 or lower and an Abbe number (vd) to the d-linefalling within a range of 43 or higher and 46 or lower, and yet has aliquidus temperature of 1200° C. or lower.

This and other objects and features of this invention will become clearfrom the following description, taken in conjunction with the preferredembodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Embodiment I-1)

First, an optical glass composition according to Embodiment I-1 of thepresent invention is described below in detail. This optical glasscomposition has the following composition.

That is, the optical glass composition according to the presentEmbodiment I-1 contains, in % by mole, 0% or more and 25.0% or less ofSiO₂, 20.0% or more and 40.0% or less of B₂O₃, 0% or more and 5.0% orless of Li₂O, 3.0% or more and 15.0% or less of ZnO, 0% or more and10.0% or less of ZrO₂, 2.0% or more and 7.0% or less of Ta₂O₅, 6.0% ormore and 25.0% or less of La₂O₃, 5.0% or more and 22.0% or less ofGd₂O₃, 66.5% or less of La₂O₃+Gd₂O₃+B₂O₃ and 26.0% or more ofLa₂O₃+Gd₂O₃. From this optical glass composition, more stable highrefractive index-lower to middle dispersion type optical glass isobtained that has a refractive index (nd) to the d-line of 1.83 orhigher and 1.86 or lower and an Abbe number (vd) to the d-line of 43 orhigher and 46 or lower, and yet has a liquidus temperature of 1300° C.or lower.

Next, the individual components contained in the optical glasscomposition are described below in detail. Hereinafter, the contents ofthe individual components are expressed in % by mole.

SiO₂ serves as a component for composing a network, and is a componentfor improving devitrification resistance. Nevertheless, when anexcessive amount of SiO₂ is used, its solubility becomes poor, and hencedifficulty arises in stable preparing. Thus, the amount of SiO₂ is setto be 0% or more and 25.0% or less and, preferably, 0% or more and 23.0%or less. Here, in order to prevent the devitrification resistance frombecoming poor and the glass from becoming unstable, it is preferablethat the amount of SiO₂ is 13.0% or more.

B₂O₃ serves as a component for composing a network, and has the effectof lowering a temperature range necessary for ensuring a desired meltingproperty and a desired viscous flow. Nevertheless, when the amount ofB₂O₃ exceeds 40.0%, the refractive index becomes excessively low. Incontrast, when the amount of B₂O₃ is less than 20.0%, the temperaturerange necessary for ensuring a desired melting property and a desiredfluidity becomes excessively high. A preferable amount of B₂O₃ is 25.0%or more and 36.0% or less.

Li₂O has the effect of lowering glass transition temperature (denoted byTg, hereinafter) so as to improve the melting property. Nevertheless,when an excessive amount of Li₂O is used, remarkable degradation arisesin the devitrification resistance and the refractive index. Thus, theamount of Li₂O is set to be 0% or more and 5.0% or less and, preferably,0% or more and 3.0% or less. Here, in order that Tg should be lowered sothat the effect of improving the melting property should be achievedmore successfully, it is preferable that the amount of Li₂O is 0.5% ormore.

Similarly to Li₂O, K₂O and Na₂O have the effect of lowering Tg so as toimprove the melting property. Nevertheless, the use of K₂O and Na₂Ocauses a possibility that remarkable degradation of the devitrificationresistance and the refractive index is accelerated. Thus, when K₂O andNa₂O need be used, the amount of each is set to be 0% or more and 6.0%or less.

ZnO has the effects of improving the devitrification resistance andlowering the temperature necessary for viscous flow. Nevertheless, whenthe amount of ZnO exceeds 15.0%, difficulty arises in adjusting therefractive index (nd) and the Abbe number (vd) into desired ranges. Incontrast, when the amount of ZnO is less than 3.0%, the effects ofimproving the devitrification resistance and lowering the temperaturenecessary for viscous flow become insufficient. A preferable amount ofZnO is 6.0% or more and 14.0% or less.

ZrO₂ has the effects of improving the refractive index and alsoimproving the devitrification resistance. Nevertheless, when anexcessive amount of ZrO₂ is used, the devitrification resistance becomespoor and so does the solubility. The amount of ZrO₂ is 0% or more and10.0% or less. Here, in order that the effects of improving therefractive index and also improving the devitrification resistanceshould be achieved more successfully, it is preferable that the amountof ZrO₂ is 1.0% or more.

Ta₂O₅ improves the refractive index, and is one of the components thatcontrol the Abbe number. Nevertheless, when the amount of Ta₂O₅ exceeds7.0%, the melting property becomes poor, and hence difficulty arises inpreparing. In contrast, when the amount of Ta₂O₅ is less than 2.0%, theeffect of improving the refractive index becomes insufficient. Apreferable amount of Ta₂O₅ is 2.2% or more and 4.0% or less.

Similarly to Ta₂O₅, La₂O₃ improves the refractive index, and is one ofthe most important components that control the Abbe number. When theamount of La₂O₃ is less than 6.0%, difficulty arises in adjusting theAbbe number into a desired range. In contrast, when the amount of La₂O₃exceeds 25.0%, the devitrification resistance becomes poor. Thus, glassbecomes unstable, and hence difficulty arises in preparing. A preferableamount of La₂O₃ is 11.0% or more and 22.0% or less.

Similarly to La₂O₃, Gd₂O₃ improves the refractive index, and is one ofthe components that control the Abbe number. Nevertheless, when theamount of Gd₂O₃ exceeds 22.0%, the devitrification resistance becomespoor. Thus, glass becomes unstable, and hence difficulty arises inpreparing. In contrast, the amount of Gd₂O₃ is less than 5.0%, theeffect of improving the refractive index becomes insufficient. Apreferable amount of Gd₂O₃ is 9.0% or more and 19.0% or less.

Here, in order that the liquidus temperature should be decreased to adesired range, the total amount of La₂O₃, Gd₂O₃ and B₂O₃(La₂O₃+Gd₂O₃+B₂O₃) is adjusted into 66.5% or less. Here, when the totalamount is too small, improvement of the refractive index becomesinsufficient, and lowering of a temperature range necessary for ensuringa desired melting property and a desired viscous flow becomesinsufficient. Thus, the total amount is adjusted preferably into 50.0%or more.

Here, in order that the Abbe number should be increased to a desiredrange, the total amount of La₂O₃ and Gd₂O₃ (La₂O₃+Gd₂O₃) is adjustedinto 26.0% or more and, preferably, into 28.0% or more. Here, when thetotal amount is excessive, the devitrification resistance becomes poor.Thus, the total amount is adjusted preferably into 35.0% or less.

GeO₂ may be used as a replacement of SiO₂, and serves as a component forcomposing a network. Nevertheless, when an excessive amount of GeO₂ isused, this causes a possibility that the devitrification resistancebecomes poor. Thus, it is preferable that the amount of GeO₂ is 0% ormore and 16.0% or less and, more preferably, 0% or more and 8.0% orless.

BaO is a component that improves preparing property, and may be usedwithin a range of 0% or more and 6.0% or less. Here, alkaline earthmetal oxides R′O (here, R′ is at least one of Sr, Ca and Mg) other thanthe BaO have a tendency that when an excessive amount is used, thedevitrification resistance becomes poor. Thus, non-use of these ispreferable. Accordingly, when the use of R′O is unavoidable, it ispreferable that their total amount is set to be 10.0% or less.

Similarly to La₂O₃, Nb₂O₅ improves the refractive index, and is one ofthe components that control the Abbe number. Further, when La₂O₃ isreplaced by Nb₂O₅, the effect of improving the devitrificationresistance is also obtained. Nevertheless, when an excessive amount ofNb₂O₅ is used, difficulty arises in adjusting the Abbe number into adesired range. Thus, the amount of Nb₂O₅ is set to be 0% or more and3.0% or less and, preferably, 0% or more and 2.0% or less. Here, inorder that the effect of improving the refractive index and thedevitrification resistance should be achieved more successfully, it ispreferable that the amount of Nb₂O₅ is 0.5% or more.

TiO₂ has the effects of controlling the refractive index and the Abbenumber and improving the devitrification resistance. Nevertheless, whenan excessive amount of TiO₂ is used, difficulty arises in adjusting theAbbe number into a desired range. Thus, the amount of TiO₂ is set to be0% or more and 3.0% or less and, preferably, 0% or more and 2.0% orless. Here, in order that the effect of improving the refractive indexand the devitrification resistance should be achieved more successfully,it is preferable that the amount of TiO₂ is 0.5% or more.

Similarly to La₂O₃, Y₂O₃ and Yb₂O₃ improve the refractive index, and arecomponents that control the Abbe number. Nevertheless, when an excessiveamount of these Y₂O₃ and Yb₂O₃ is used, the devitrification resistancebecomes poor. Thus, glass becomes unstable, and hence difficulty arisesin preparing. Thus, when La₂O₃ need be replaced by these Y₂O₃ and Yb₂O₃,it is preferable that the amount of each is set to be 0% or more and3.0% or less.

WO₃ is a component for alleviating the high devitrification caused byLa₂O₃ and adjusting the refractive index and the Abbe number intodesired ranges. Nevertheless, when an excessive amount of WO₃ is used,this causes a possibility that the transmissivity in a blue light rangebecomes poor. Thus, the amount of WO₃ is 0% or more and 3.0% or lessand, preferably, 0% or more and 2.0% or less. Here, in order that theeffects of alleviating the high devitrification caused by La₂O₃ andadjusting the refractive index and the Abbe number into desired rangesshould be achieved more successfully, it is preferable that the amountof WO₃ is 0.5% or more.

Al₂O₃ may be used for adjusting the refractive index. However, it ispreferable that the amount is 0% or more and 10% or less. Further, inorder to adjust the refractive index, Ga₂O₃ and In₂O₃ may be used in anamount up to approximately 10% each. Nevertheless, the use of thesecauses a possibility that the devitrification resistance becomes poor.Thus, it is preferable that these are not used in an excessive amount.

In addition to the above-mentioned components, Sb₂O₃ and SnO₂ may beused which are generally used as fining agents. Here, it is preferablethat the amount of Sb₂O₃ and SnO₂ is 0% or more and 2% or less each.Nevertheless, As₂O₃ having a strong effect as a fining agent is toxic.Thus, it is preferable that As₂O₃ is not used.

In addition, as for Pb and its compounds, compounds including Te, Se, orCd, as well as radioactive substances such as compounds including U orTh, it is preferable that they are not used from the viewpoint ofsafety. Further, it is also preferable that substances such as compoundsincluding Cu, Cr, V, Fe, Ni or Co that cause coloring are not used.

When the individual components are adjusted into the above-mentionedratios, a high refractive index-lower to middle dispersion type opticalglass composition is obtained that has a refractive index (nd) to thed-line of 1.83 or higher and 1.86 or lower and an Abbe number (vd) tothe d-line of 43 or higher and 46 or lower, and yet has a liquidustemperature of 1300° C. or lower.

Here, when the glass is handled in a softened or molten state, from theviewpoints of melting property, processability (droplet property),crystallinity and the like, a lower liquidus temperature is generallypreferable in the optical glass. Accordingly, as mentioned above, theoptical glass composition according to the present Embodiment I-1 has aliquidus temperature of 1300° C. or lower and, preferably, 1295° C. orlower.

(Embodiment I-2)

Next, a preform according to Embodiment I-2 of the present invention anda producing method for the same are described below in detail. Thepreform is softened by heating so as to be used at least for pressmolding. Thus, the weight and the shape of the preform are determinedappropriately in accordance with the size and the shape of a pressmolded article serving as a final target. The preform according to thepresent Embodiment I-2 is formed from the optical glass compositionaccording to the above-mentioned Embodiment I-1, and hence obtainedwithout losing various features of the optical glass composition inEmbodiment I-1.

The preform produced from the optical glass composition according toEmbodiment I-1 is described below. Here, the preform is a preliminaryglass form that is heated and used for precision press molding. Thepreforms are divided into a gob preform produced by forming a moltenglass material and a polished preform produced by physically polishing aglass material. The optical glass composition according to EmbodimentI-1 can be used for both of the gob preform and the polished preform.

The producing method for the preform is described below. First, glassraw materials (the above-mentioned individual components) for theoptical glass composition according to the above-mentioned EmbodimentI-1 are weighed and prepared, and then processed in processing stepssuch as dissolution, defoaming, fining and homogenization so thathomogeneous molten glass is obtained which contains no foreignsubstances therein. Then, at the time when the molten glass is caused toflow out through an outflow pipe (referred to as a nozzle, hereinafter)made of platinum alloy or the like, a temperature condition for thevicinity of the nozzle is set up strictly into a range of not causingdevitrification in the glass. The molten glass flowing out is cast intoa receiving die having a receiving surface of planar shape, concaveshape, convex shape or the like or into a molding die in which anenclosure is provided in the circumference of a planar surface, aconcave surface or a convex surface. As a result, the molten glass isformed into a desired shape. In the following description, preferableforming methods are illustrated.

A first forming method is one example of producing methods for the gobpreform. First, a molten glass lump having a weight corresponding to afinal molded article or alternatively a desired weight including anaddition necessary for secondary processing into a final molded articleis dropped into each of a plurality of receiving dies arranged under thenozzle. Then, cooling is performed simultaneously to the forming ofglass lumps so that gob preforms are obtained.

A second forming method is another example of producing methods for thegob preform. The second forming method is suitable for a case that apreform having a relatively heavy weight is to be produced. First, thetip of the molten glass flowing out through the nozzle is brought intocontact with the receiving die surface. Then, at each time when adesired weight is reached, the receiving die is rapidly separated fromthe molten glass so that the molten glass is cut off. As a result, theforming of glass free from striae and shear marks is achieved. Further,when necessary, press molding may be performed with aimingsimultaneously at cooling of the molten glass lump, so that a desiredshape may be imparted and gob preforms are obtained.

A third forming method is one example of producing methods for thepolished preform. First, a glass lump having a desired shape is producedby a similar method to the first and the second forming methods. At thistime, the glass lump having a desired shape has a total weight of adesired weight plus an additional weight necessary for finishing all thesurfaces including optically functional surfaces of a final product(such as a lens) by means of machining. Then, the glass lump having adesired shape is cut and polished by means of machining so that polishedpreforms are obtained.

The preform obtained by each of the first and the second forming methodsdescribed above can be used as a preform for press molding directly inprecision press molding. Further, the third forming method provides apreform for polish. In order that breakage should be avoided in thepreform at the time of handling, for example, a three-dimensionalcooling method, an optimal cooling rate, an annealing treatment and thelike may be selected depending on the shape and the weight.

As described above, a preform for press molding and a preform for polishhaving a desired weight can be obtained from the optical glasscomposition according to Embodiment I-1. In the case of a preform forpress molding, for the purpose of mold release at the time of forming,it is preferable that the surface roughness of the receiving die surfaceis adjusted or alternatively that a mold-release film is formed. In thecase of a preform for polish, when an HBN (boron-containingmold-releasing agent) is applied, releasing from the die becomes easier.Thus, this approach is preferable.

(Embodiment I-3)

Next, an optical element according to Embodiment I-3 of the presentinvention is described below. The optical element according toEmbodiment I-3 has optical indices determined by the composition of theoptical glass composition according to the above-mentioned EmbodimentI-1. That is, the refractive index (nd) to the d-line is 1.83 to 1.86,while the Abbe number (vd) to the d-line is 43 to 46, and while theliquidus temperature is 1300° C. or lower. Further, this optical elementhas also a feature that optical absorption by coloring is low in avisible light range. The optical element employing the optical glasscomposition according to Embodiment I-1 is an optical element suitablefor an optical system in a digital camera, a video camera, a mobiledevice or the like.

Examples of the optical element according to the present Embodiment I-3include a spherical lens, an aspheric lens and a micro lens, as well asa prism and a diffraction grating. Other examples include an opticalelement cemented with an optical element composed of a glass material oran optical material of another kind.

Next, a producing method for the optical element according to EmbodimentI-3 is described below. The optical element according to Embodiment I-3can be produced by supplying the preform according to theabove-mentioned Embodiment I-2 into a molding die, then softening it byheating, then performing press molding, and then performing polish whennecessary.

Press molding methods employable for obtaining the optical element aredivided into two typical methods. These methods are selected dependingon the means of forming optically functional surfaces where light entersand exits.

First means is means referred to as precision press molding. The moldingsurfaces of a press molding die are precisely processed in advance intoreversal shapes of the optically functional surfaces of the opticalelement serving as a final molded article. Then, when necessary, amold-release film is provided in order to avoid fusion between thepreform and the molding die. Then, press molding is performed so thatthe shape of the above-mentioned molding surface is preciselytransferred to the preform for press molding having been softened byheating. According to this means, grinding and polishing of opticallyfunctional surfaces are unnecessary. That is, the optical element can beproduced solely by press molding. The press molding is performed in aninert atmosphere like in nitrogen gas. Here, when a preform for pressmolding having an additional weight relative to the final molded articleis used, for example, in the case of a lens, centering may be performedby grinding by the amount corresponding to the additional weight.

Second means is that press molding is performed using a preform forpolish having a shape which is similar to the shape of the opticalelement serving as a final molded article and which is larger than theoptical element. The formed press molded article contains opticallyfunctional surfaces, and the surfaces of the optical element are formedby machining. In the press molded article, in order that breakage thatcould be caused by the machining should be avoided in the glass,residual strain need be minimized. Further, also in order that requiredoptical indices should be achieved, an appropriate annealing treatmentis necessary. According to this means, press molding may be performed inan ordinary atmosphere. Further, the above-mentioned mold-releasingagent may be used.

Here, in both cases regardless of whether the first means or the secondmeans described above is selected, the refractive index (nd) and theAbbe number (vd) of the obtained optical element varies slightly owingto the heat history in the producing process. Thus, when an opticalelement having precisely specified optical indices is to be produced,component adjustment for the optical glass composition, heat historyadjustment in the producing process, adjustment of incorporating theamount of variation into the optical design when necessary, or the likemay be selected appropriately with taking into consideration theabove-mentioned variation in the refractive index (nd) and the Abbenumber (vd). As a result, an optical element is obtained that hasdesired optical indices and an excellent transmissivity and that isparticularly suitable as an optical component of a device whichincorporates a solid-state image sensor or the like.

(Embodiment II-1)

First, an optical glass composition according to Embodiment II-1 of thepresent invention is described below in detail. This optical glasscomposition has the following composition.

That is, the optical glass composition according to the presentEmbodiment II-1 contains, in % by mole, 5.0% or more and 25.0% or lessof SiO₂, 25.0% or more and 40.0% or less of B₂O₃, 10.0% or more and15.0% or less of ZnO, 0% or more and 5.0% or less of ZrO₂, 10.0% or moreand 25.0% or less of La₂O₃, 5.0% or more and 20.0% or less of Gd₂O₃ and0% or more and 5.0% or less of Ta₂O₅, and in this optical glasscomposition, La₂O₃/Gd₂O₃ is, in molar ratio, more than 0 and less than3.5. Also, this optical glass composition substantially contains noLi₂O. From this optical glass composition, more stable high refractiveindex-lower to middle dispersion type optical glass is obtained that hasa refractive index (nd) to the d-line of 1.83 or higher and 1.87 orlower and an Abbe number (vd) to the d-line of 43 or higher and 47 orlower, and yet has a liquidus temperature of 1300° C. or lower.

Next, the individual components contained in the optical glasscomposition are described below in detail. Hereinafter, the contents ofthe individual components are expressed in % by mole.

SiO₂ serves as a component for composing a network, and is an essentialcomponent for improving devitrification resistance. Nevertheless, whenthe amount of SiO₂ exceeds 25.0%, its solubility becomes poor, and hencedifficulty arises in stable preparing. Further, its liquidus temperaturegoes high, and hence difficulty arises in preparing. In contrast, whenthe amount of SiO₂ is less than 5.0%, the devitrification resistancebecomes poor, and hence the glass becomes unstable. A preferable amountof SiO₂ is 8.0% or more and 23.0% or less.

B₂O₃ serves as a component for composing a network, and has the effectof lowering a temperature range necessary for ensuring a desired meltingproperty and a desired viscous flow. Nevertheless, when the amount ofB₂O₃ exceeds 40.0%, the refractive index becomes excessively low. Incontrast, when the amount of B₂O₃ is less than 25.0%, the temperaturerange necessary for ensuring a desired melting property and a desiredfluidity becomes excessively high. A preferable amount of B₂O₃ is 26.0%or more and 39.0% or less.

ZnO has the effects of improving the devitrification resistance andlowering the temperature necessary for viscous flow. Nevertheless, whenthe amount of ZnO exceeds 15.0%, difficulty arises in adjusting therefractive index (nd) and the Abbe number (vd) into desired ranges. Incontrast, when the amount of ZnO is less than 10.0%, the effects ofimproving the devitrification resistance and lowering the temperaturenecessary for viscous flow become insufficient. A preferable amount ofZnO is 13.0% or more and 14.5% or less.

ZrO₂ has the effects of improving the refractive index and alsoimproving the devitrification resistance. Nevertheless, when anexcessive amount of ZrO₂ is used, the devitrification resistance becomespoor and so does the solubility. The amount of ZrO₂ is 0% or more and5.0% or less, preferably, 0% or more and 3.0% or less. Here, in orderthat the effects of improving the refractive index and also improvingthe devitrification resistance should be achieved more successfully, itis preferable that the amount of ZrO₂ is 1.0% or more.

La₂O₃ improves the refractive index, and is one of the most importantcomponents that control the Abbe number. When the amount of La₂O₃ isless than 10.0%, difficulty arises in adjusting the Abbe number into adesired range. In contrast, when the amount of La₂O₃ exceeds 25.0%, thedevitrification resistance becomes poor. Thus, glass becomes unstable,and hence difficulty arises in preparing. A preferable amount of La₂O₃is 11.0% or more and 22.0% or less.

Similarly to La₂O₃, Gd₂O₃ improves the refractive index, and is one ofthe components that control the Abbe number. Nevertheless, when theamount of Gd₂O₃ exceeds 20.0%, the devitrification resistance becomespoor. Thus, glass becomes unstable, and hence difficulty arises inpreparing. In contrast, the amount of Gd₂O₃ is less than 5.0%, theeffect of improving the refractive index becomes insufficient. Apreferable amount of Gd₂O₃ is 9.0% or more and 19.0% or less.

Similarly to La₂O₃, Ta₂O₅ improves the refractive index, and is one ofthe components that control the Abbe number. Nevertheless, when anexcessive amount of Ta₂O₅ is used, the melting property becomes poor,and hence difficulty arises in preparing. Thus, the amount of Ta₂O₅ isset to be 0% or more and 5.0% or less and, preferably, 0% or more and4.0% or less. Here, in order that the effect of improving the refractiveindex should be achieved more successfully, it is preferable that theamount of Ta₂O₅ is 1.0% or more.

Here, in order to prevent the devitrification resistance from becomingpoor, the molar ratio of La₂O₃ to Gd₂O₃, that is “La₂O₃/Gd₂O₃”, isadjusted into less than 3.5, preferably, into 2.5 or less. Here, themolar ratio is more than 0. In contrast, when the amount of La₂O₃ is toosmall, there is a possibility that the devitrification resistancebecomes poor. Accordingly, in order that the effect of thedevitrification resistance should be achieved more successfully, it ispreferable that the molar ratio is 0.5 or more.

Li₂O has the effect of lowering Tg so as to improve the meltingproperty. Nevertheless, Li₂O prevents the liquidus temperature fromlowering, and degrades the devitrification resistance and the refractiveindex. Accordingly, the optical glass composition according to thepresent Embodiment II-1 does not contain Li₂O substantially.

Similarly to Li₂O, K₂O and Na₂O have the effect of lowering Tg so as toimprove the melting property. Nevertheless, the use of K₂O and Na₂Ocauses a possibility that remarkable degradation of the devitrificationresistance and the refractive index is accelerated. Thus, when K₂O andNa₂O need be used, the amount of each is set to be 0% or more and 6.0%or less.

GeO₂ may be used as a replacement of SiO₂, and serves as a component forcomposing a network. Nevertheless, when an excessive amount of GeO₂ isused, this causes a possibility that the devitrification resistancebecomes poor. Thus, it is preferable that the amount of GeO₂ is 0% ormore and 16.0% or less and, more preferably, 0% or more and 8.0% orless.

BaO is a component that improves preparing property, and may be usedwithin a range of 0% or more and 6.0% or less. Here, alkaline earthmetal oxides R′O (here, R′ is at least one of Sr, Ca and Mg) other thanthe BaO have a tendency that when an excessive amount is used, thedevitrification resistance becomes poor. Thus, non-use of these ispreferable. Accordingly, when the use of R′O is unavoidable, it ispreferable that their total amount is set to be 10.0% or less.

Similarly to La₂O₃, Nb₂O₅ improves the refractive index, and is one ofthe components that control the Abbe number. Further, when La₂O₃ isreplaced by Nb₂O₅, the effect of improving the devitrificationresistance is also obtained. Nevertheless, when an excessive amount ofNb₂O₅ is used, difficulty arises in adjusting the Abbe number into adesired range. Thus, the amount of Nb₂O₅ is set to be 0% or more and3.0% or less and, preferably, 0% or more and 2.0% or less. Here, inorder that the effect of improving the refractive index and thedevitrification resistance should be achieved more successfully, it ispreferable that the amount of Nb₂O₅ is 0.5% or more.

TiO₂ has the effects of controlling the refractive index and the Abbenumber and improving the devitrification resistance. Nevertheless, whenan excessive amount of TiO₂ is used, difficulty arises in adjusting theAbbe number into a desired range. Thus, the amount of TiO₂ is set to be0% or more and 3.0% or less and, preferably, 0% or more and 2.0% orless. Here, in order that the effect of improving the refractive indexand the devitrification resistance should be achieved more successfully,it is preferable that the amount of TiO₂ is 0.5% or more.

Similarly to La₂O₃, Y₂O₃ and Yb₂O₃ improve the refractive index, and arecomponents that control the Abbe number. Nevertheless, when an excessiveamount of these Y₂O₃ and Yb₂O₃ is used, the devitrification resistancebecomes poor. Thus, glass becomes unstable, and hence difficulty arisesin preparing. Thus, when La₂O₃ need be replaced by these Y₂O₃ and Yb₂O₃,it is preferable that the amount of each is set to be 0% or more and3.0% or less.

WO₃ is a component for alleviating the high devitrification caused byLa₂O₃ and adjusting the refractive index and the Abbe number intodesired ranges. Nevertheless, when an excessive amount of WO₃ is used,this causes a possibility that the transmissivity in a blue light rangebecomes poor. Thus, the amount of WO₃ is 0% or more and 3.0% or lessand, preferably, 0% or more and 2.0% or less. Here, in order that theeffects of alleviating the high devitrification caused by La₂O₃ andadjusting the refractive index and the Abbe number into desired rangesshould be achieved more successfully, it is preferable that the amountof WO₃ is 0.5% or more.

Al₂O₃ may be used for adjusting the refractive index. However, it ispreferable that the amount is 0% or more and 10% or less. Further, inorder to adjust the refractive index, Ga₂O₃ and In₂O₃ may be used in anamount up to approximately 10% each. Nevertheless, the use of thesecauses a possibility that the devitrification resistance becomes poor.Thus, it is preferable that these are not used in an excessive amount.

In addition to the above-mentioned components, Sb₂O₃ and SnO₂ may beused which are generally used as fining agents. Here, it is preferablethat the amount of Sb₂O₃ and SnO₂ is 0% or more and 2% or less each.Nevertheless, As₂O₃ having a strong effect as a fining agent is toxic.Thus, it is preferable that As₂O₃ is not used.

In addition, as for Pb and its compounds, compounds including Te, Se, orCd, as well as radioactive substances such as compounds including U orTh, it is preferable that they are not used from the viewpoint ofsafety. Further, it is also preferable that substances such as compoundsincluding Cu, Cr, V, Fe, Ni or Co that cause coloring are not used.

When the individual components are adjusted into the above-mentionedratios, a high refractive index-lower to middle dispersion type opticalglass composition is obtained that has a refractive index (nd) to thed-line of 1.83 or higher and 1.87 or lower and an Abbe number (vd) tothe d-line of 43 or higher and 47 or lower, and yet has a liquidustemperature of 1300° C. or lower.

Here, when the glass is handled in a softened or molten state, from theviewpoints of melting property, processability (droplet property),crystallinity and the like, a lower liquidus temperature is generallypreferable in the optical glass. Accordingly, as mentioned above, theoptical glass composition according to the present Embodiment II-1 has aliquidus temperature of 1300° C. or lower and, preferably, 1295° C. orlower.

(Embodiment II-2)

Next, a preform according to Embodiment II-2 of the present inventionand a producing method for the same are described below in detail. Thepreform is softened by heating so as to be used at least for pressmolding. Thus, the weight and the shape of the preform are determinedappropriately in accordance with the size and the shape of a pressmolded article serving as a final target. The preform according to thepresent Embodiment II-2 is formed from the optical glass compositionaccording to the above-mentioned Embodiment II-1, and hence obtainedwithout losing various features of the optical glass composition inEmbodiment II-1.

The preform produced from the optical glass composition according toEmbodiment II-1 is described below. Here, the preform is a preliminaryglass form that is heated and used for precision press molding. Thepreforms are divided into a gob preform produced by forming a moltenglass material and a polished preform produced by physically polishing aglass material. The optical glass composition according to EmbodimentII-1 can be used for both of the gob preform and the polished preform.

The producing method for the preform is described below. First, glassraw materials (the above-mentioned individual components) for theoptical glass composition according to the above-mentioned EmbodimentII-1 are weighed and prepared, and then processed in processing stepssuch as dissolution, defoaming, fining and homogenization so thathomogeneous molten glass is obtained which contains no foreignsubstances therein. Then, at the time when the molten glass is caused toflow out through an outflow pipe (referred to as a nozzle, hereinafter)made of platinum alloy or the like, a temperature condition for thevicinity of the nozzle is set up strictly into a range of not causingdevitrification in the glass. The molten glass flowing out is cast intoa receiving die having a receiving surface of planar shape, concaveshape, convex shape or the like or into a molding die in which anenclosure is provided in the circumference of a planar surface, aconcave surface or a convex surface. As a result, the molten glass isformed into a desired shape. In the following description, preferableforming methods are illustrated.

A first forming method is one example of producing methods for the gobpreform. First, a molten glass lump having a weight corresponding to afinal molded article or alternatively a desired weight including anaddition necessary for secondary processing into a final molded articleis dropped into each of a plurality of receiving dies arranged under thenozzle. Then, cooling is performed simultaneously to the forming ofglass lumps so that gob preforms are obtained.

A second forming method is another example of producing methods for thegob preform. The second forming method is suitable for a case that apreform having a relatively heavy weight is to be produced. First, thetip of the molten glass flowing out through the nozzle is brought intocontact with the receiving die surface. Then, at each time when adesired weight is reached, the receiving die is rapidly separated fromthe molten glass so that the molten glass is cut off. As a result, theforming of glass free from striae and shear marks is achieved. Further,when necessary, press molding may be performed with aimingsimultaneously at cooling of the molten glass lump, so that a desiredshape may be imparted and gob preforms are obtained.

A third forming method is one example of producing methods for thepolished preform. First, a glass lump having a desired shape is producedby a similar method to the first and the second forming methods. At thistime, the glass lump having a desired shape has a total weight of adesired weight plus an additional weight necessary for finishing all thesurfaces including optically functional surfaces of a final product(such as a lens) by means of machining. Then, the glass lump having adesired shape is cut and polished by means of machining so that polishedpreforms are obtained.

The preform obtained by each of the first and the second forming methodsdescribed above can be used as a preform for press molding directly inprecision press molding. Further, the third forming method provides apreform for polish. In order that breakage should be avoided in thepreform at the time of handling, for example, a three-dimensionalcooling method, an optimal cooling rate, an annealing treatment and thelike may be selected depending on the shape and the weight.

As described above, a preform for press molding and a preform for polishhaving a desired weight can be obtained from the optical glasscomposition according to Embodiment II-1. In the case of a preform forpress molding, for the purpose of mold release at the time of forming,it is preferable that the surface roughness of the receiving die surfaceis adjusted or alternatively that a mold-release film is formed. In thecase of a preform for polish, when an HBN (boron-containingmold-releasing agent) is applied, releasing from the die becomes easier.Thus, this approach is preferable.

(Embodiment II-3)

Next, an optical element according to Embodiment II-3 of the presentinvention is described below. The optical element according toEmbodiment II-3 has optical indices determined by the composition of theoptical glass composition according to the above-mentioned EmbodimentII-1. That is, the refractive index (nd) to the d-line is 1.83 to 1.87,while the Abbe number (vd) to the d-line is 43 to 47, and while theliquidus temperature is 1300° C. or lower. Further, this optical elementhas also a feature that optical absorption by coloring is low in avisible light range. The optical element employing the optical glasscomposition according to Embodiment II-1 is an optical element suitablefor an optical system in a digital camera, a video camera, a mobiledevice or the like.

Examples of the optical element according to the present Embodiment II-3include a spherical lens, an aspheric lens and a micro lens, as well asa prism and a diffraction grating. Other examples include an opticalelement cemented with an optical element composed of a glass material oran optical material of another kind.

Next, a producing method for the optical element according to EmbodimentII-3 is described below. The optical element according to EmbodimentII-3 can be produced by supplying the preform according to theabove-mentioned Embodiment II-2 into a molding die, then softening it byheating, then performing press molding, and then performing polish whennecessary.

Press molding methods employable for obtaining the optical element aredivided into two typical methods. These methods are selected dependingon the means of forming optically functional surfaces where light entersand exits.

First means is means referred to as precision press molding. The moldingsurfaces of a press molding die are precisely processed in advance intoreversal shapes of the optically functional surfaces of the opticalelement serving as a final molded article. Then, when necessary, amold-release film is provided in order to avoid fusion between thepreform and the molding die. Then, press molding is performed so thatthe shape of the above-mentioned molding surface is preciselytransferred to the preform for press molding having been softened byheating. According to this means, grinding and polishing of opticallyfunctional surfaces are unnecessary. That is, the optical element can beproduced solely by press molding. The press molding is performed in aninert atmosphere like in nitrogen gas. Here, when a preform for pressmolding having an additional weight relative to the final molded articleis used, for example, in the case of a lens, centering may be performedby grinding by the amount corresponding to the additional weight.

Second means is that press molding is performed using a preform forpolish having a shape which is similar to the shape of the opticalelement serving as a final molded article and which is larger than theoptical element. The formed press molded article contains opticallyfunctional surfaces, and the surfaces of the optical element are formedby machining. In the press molded article, in order that breakage thatcould be caused by the machining should be avoided in the glass,residual strain need be minimized. Further, also in order that requiredoptical indices should be achieved, an appropriate annealing treatmentis necessary. According to this means, press molding may be performed inan ordinary atmosphere. Further, the above-mentioned mold-releasingagent may be used.

Here, in both cases regardless of whether the first means or the secondmeans described above is selected, the refractive index (nd) and theAbbe number (vd) of the obtained optical element varies slightly owingto the heat history in the producing process. Thus, when an opticalelement having precisely specified optical indices is to be produced,component adjustment for the optical glass composition, heat historyadjustment in the producing process, adjustment of incorporating theamount of variation into the optical design when necessary, or the likemay be selected appropriately with taking into consideration theabove-mentioned variation in the refractive index (nd) and the Abbenumber (vd). As a result, an optical element is obtained that hasdesired optical indices and an excellent transmissivity and that isparticularly suitable as an optical component of a device whichincorporates a solid-state image sensor or the like.

(Embodiment III-1)

First, an optical glass composition according to Embodiment III-1 of thepresent invention is described below in detail. This optical glasscomposition has the following composition.

That is, the optical glass composition according to the presentEmbodiment III-1 contains, in % by mole, 5.0% or more and 25.0% or lessof SiO₂, 20.0% or more and 40.0% or less of B₂O₃, 0% or more and 15.0%or less of ZnO, 0% or more and 5.0% or less of ZrO₂, 10.0% or more and25.0% or less of La₂O₃, 0% or more and 5.0% or less of Ta₂O₅ and 5.0% ormore and 20.0% or less of Gd₂O₃, and in this optical glass composition,SiO₂/B₂O₃ is, in molar ratio, 0.25 or more and 0.90 or less. Also, thisoptical glass composition substantially contains no Li₂O. From thisoptical glass composition, more stable high refractive index-lower tomiddle dispersion type optical glass is obtained that has a refractiveindex (nd) to the d-line of 1.83 or higher and 1.87 or lower and an Abbenumber (vd) to the d-line of 43 or higher and 47 or lower, and yet has aliquidus temperature of 1300° C. or lower.

Next, the individual components contained in the optical glasscomposition are described below in detail. Hereinafter, the contents ofthe individual components are expressed in % by mole.

SiO₂ serves as a component for composing a network, and is an essentialcomponent for improving devitrification resistance. Nevertheless, whenthe amount of SiO₂ exceeds 25.0%, its solubility becomes poor, and hencedifficulty arises in stable preparing. Further, its liquidus temperaturegoes high, and hence difficulty arises in preparing. In contrast, whenthe amount of SiO₂ is less than 5.0%, the devitrification resistancebecomes poor, and hence the glass becomes unstable. A preferable amountof SiO₂ is 8.0% or more and 23.0% or less.

B₂O₃ serves as a component for composing a network, and has the effectof lowering a temperature range necessary for ensuring a desired meltingproperty and a desired viscous flow. Nevertheless, when the amount ofB₂O₃ exceeds 40.0%, the refractive index becomes excessively low. Incontrast, when the amount of B₂O₃ is less than 20.0%, the temperaturerange necessary for ensuring a desired melting property and a desiredfluidity becomes excessively high. A preferable amount of B₂O₃ is 26.0%or more and 39.0% or less.

ZnO has the effects of improving the devitrification resistance andlowering the temperature necessary for viscous flow. Nevertheless, whenan excessive amount of ZnO is used, difficulty arises in adjusting therefractive index (nd) and the Abbe number (vd) into desired ranges. Theamount of ZnO is 0% or more and 15.0% or less and, preferably, 0% ormore and 14.5% or less. Here, in order that the effects of improving thedevitrification resistance and lowering the temperature necessary forviscous flow should be achieved more successfully, it is preferable thatthe amount of ZnO is 1.0% or more.

ZrO₂ has the effects of improving the refractive index and alsoimproving the devitrification resistance. Nevertheless, when anexcessive amount of ZrO₂ is used, the devitrification resistance becomespoor and so does the solubility. The amount of ZrO₂ is 0% or more and5.0% or less, preferably, 0% or more and 3.0% or less. Here, in orderthat the effects of improving the refractive index and also improvingthe devitrification resistance should be achieved more successfully, itis preferable that the amount of ZrO₂ is 1.0% or more.

La₂O₃ improves the refractive index, and is one of the most importantcomponents that control the Abbe number. When the amount of La₂O₃ isless than 10.0%, difficulty arises in adjusting the Abbe number into adesired range. In contrast, when the amount of La₂O₃ exceeds 25.0%, thedevitrification resistance becomes poor. Thus, glass becomes unstable,and hence difficulty arises in preparing. A preferable amount of La₂O₃is 11.0% or more and 22.0% or less.

Similarly to La₂O₃, Ta₂O₅ improves the refractive index, and is one ofthe components that control the Abbe number. Nevertheless, when anexcessive amount of Ta₂O₅ is used, the melting property becomes poor,and hence difficulty arises in preparing. Thus, the amount of Ta₂O₅ isset to be 0% or more and 5.0% or less and, preferably, 0% or more and4.0% or less. Here, in order that the effect of improving the refractiveindex should be achieved more successfully, it is preferable that theamount of Ta₂O₅ is 1.0% or more.

Similarly to La₂O₃, Gd₂O₃ improves the refractive index, and is one ofthe components that control the Abbe number. Nevertheless, when theamount of Gd₂O₃ exceeds 20.0%, the devitrification resistance becomespoor. Thus, glass becomes unstable, and hence difficulty arises inpreparing. In contrast, the amount of Gd₂O₃ is less than 5.0%, theeffect of improving the refractive index becomes insufficient. Apreferable amount of Gd₂O₃ is 9.0% or more and 19.0% or less.

Here, in order to prevent the devitrification resistance from becomingpoor, the molar ratio of SiO₂ to B₂O₃, that is “SiO₂/B₂O₃”, is adjustedinto 0.25 or more, preferably, into 0.27 or more. Also, in order toadjust the liquidus temperature into a desired range and to prevent thedevitrification resistance from becoming poor, the molar ratio isadjusted into 0.90 or less, preferably, into 0.80 or less.

Li₂O has the effect of lowering Tg so as to improve the meltingproperty. Nevertheless, Li₂O prevents the liquidus temperature fromlowering, and degrades the devitrification resistance and the refractiveindex. Accordingly, the optical glass composition according to thepresent Embodiment III-1 does not contain Li₂O substantially.

Similarly to Li₂O, K₂O and Na₂O have the effect of lowering Tg so as toimprove the melting property. Nevertheless, the use of K₂O and Na₂Ocauses a possibility that remarkable degradation of the devitrificationresistance and the refractive index is accelerated. Thus, when K₂O andNa₂O need be used, the amount of each is set to be 0% or more and 6.0%or less.

GeO₂ may be used as a replacement of SiO₂, and serves as a component forcomposing a network. Nevertheless, when an excessive amount of GeO₂ isused, this causes a possibility that the devitrification resistancebecomes poor. Thus, it is preferable that the amount of GeO₂ is 0% ormore and 16.0% or less and, more preferably, 0% or more and 8.0% orless.

BaO is a component that improves preparing property, and may be usedwithin a range of 0% or more and 6.0% or less. Here, alkaline earthmetal oxides R′O (here, R′ is at least one of Sr, Ca and Mg) other thanthe BaO have a tendency that when an excessive amount is used, thedevitrification resistance becomes poor. Thus, non-use of these ispreferable. Accordingly, when the use of R′O is unavoidable, it ispreferable that their total amount is set to be 10.0% or less.

Similarly to La₂O₃, Nb₂O₅ improves the refractive index, and is one ofthe components that control the Abbe number. Further, when La₂O₃ isreplaced by Nb₂O₅, the effect of improving the devitrificationresistance is also obtained. Nevertheless, when an excessive amount ofNb₂O₅ is used, difficulty arises in adjusting the Abbe number into adesired range. Thus, the amount of Nb₂O₅ is set to be 0% or more and3.0% or less and, preferably, 0% or more and 2.0% or less. Here, inorder that the effect of improving the refractive index and thedevitrification resistance should be achieved more successfully, it ispreferable that the amount of Nb₂O₅ is 0.5% or more.

TiO₂ has the effects of controlling the refractive index and the Abbenumber and improving the devitrification resistance. Nevertheless, whenan excessive amount of TiO₂ is used, difficulty arises in adjusting theAbbe number into a desired range. Thus, the amount of TiO₂ is set to be0% or more and 3.0% or less and, preferably, 0% or more and 2.0% orless. Here, in order that the effect of improving the refractive indexand the devitrification resistance should be achieved more successfully,it is preferable that the amount of TiO₂ is 0.5% or more.

Similarly to La₂O₃, Y₂O₃ and Yb₂O₃ improve the refractive index, and arecomponents that control the Abbe number. Nevertheless, when an excessiveamount of these Y₂O₃ and Yb₂O₃ is used, the devitrification resistancebecomes poor. Thus, glass becomes unstable, and hence difficulty arisesin preparing. Thus, when La₂O₃ need be replaced by these Y₂O₃ and Yb₂O₃,it is preferable that the amount of each is set to be 0% or more and3.0% or less.

WO₃ is a component for alleviating the high devitrification caused byLa₂O₃ and adjusting the refractive index and the Abbe number intodesired ranges. Nevertheless, when an excessive amount of WO₃ is used,this causes a possibility that the transmissivity in a blue light rangebecomes poor. Thus, the amount of WO₃ is 0% or more and 3.0% or lessand, preferably, 0% or more and 2.0% or less. Here, in order that theeffects of alleviating the high devitrification caused by La₂O₃ andadjusting the refractive index and the Abbe number into desired rangesshould be achieved more successfully, it is preferable that the amountof WO₃ is 0.5% or more.

Al₂O₃ may be used for adjusting the refractive index. However, it ispreferable that the amount is 0% or more and 10% or less. Further, inorder to adjust the refractive index, Ga₂O₃ and In₂O₃ may be used in anamount up to approximately 10% each. Nevertheless, the use of thesecauses a possibility that the devitrification resistance becomes poor.Thus, it is preferable that these are not used in an excessive amount.

In addition to the above-mentioned components, Sb₂O₃ and SnO₂ may beused which are generally used as fining agents. Here, it is preferablethat the amount of Sb₂O₃ and SnO₂ is 0% or more and 2% or less each.Nevertheless, As₂O₃ having a strong effect as a fining agent is toxic.Thus, it is preferable that As₂O₃ is not used.

In addition, as for Pb and its compounds, compounds including Te, Se, orCd, as well as radioactive substances such as compounds including U orTh, it is preferable that they are not used from the viewpoint ofsafety. Further, it is also preferable that substances such as compoundsincluding Cu, Cr, V, Fe, Ni or Co that cause coloring are not used.

When the individual components are adjusted into the above-mentionedratios, a high refractive index-lower to middle dispersion type opticalglass composition is obtained that has a refractive index (nd) to thed-line of 1.83 or higher and 1.87 or lower and an Abbe number (vd) tothe d-line of 43 or higher and 47 or lower, and yet has a liquidustemperature of 1300° C. or lower.

Here, when the glass is handled in a softened or molten state, from theviewpoints of melting property, processability (droplet property),crystallinity and the like, a lower liquidus temperature is generallypreferable in the optical glass. Accordingly, as mentioned above, theoptical glass composition according to the present Embodiment III-1 hasa liquidus temperature of 1300° C. or lower and, preferably, 1295° C. orlower.

(Embodiment III-2)

Next, a preform according to Embodiment III-2 of the present inventionand a producing method for the same are described below in detail. Thepreform is softened by heating so as to be used at least for pressmolding. Thus, the weight and the shape of the preform are determinedappropriately in accordance with the size and the shape of a pressmolded article serving as a final target. The preform according to thepresent Embodiment III-2 is formed from the optical glass compositionaccording to the above-mentioned Embodiment III-1, and hence obtainedwithout losing various features of the optical glass composition inEmbodiment III-1.

The preform produced from the optical glass composition according toEmbodiment III-1 is described below. Here, the preform is a preliminaryglass form that is heated and used for precision press molding. Thepreforms are divided into a gob preform produced by forming a moltenglass material and a polished preform produced by physically polishing aglass material. The optical glass composition according to EmbodimentIII-1 can be used for both of the gob preform and the polished preform.

The producing method for the preform is described below. First, glassraw materials (the above-mentioned individual components) for theoptical glass composition according to the above-mentioned EmbodimentIII-1 are weighed and prepared, and then processed in processing stepssuch as dissolution, defoaming, fining and homogenization so thathomogeneous molten glass is obtained which contains no foreignsubstances therein. Then, at the time when the molten glass is caused toflow out through an outflow pipe (referred to as a nozzle, hereinafter)made of platinum alloy or the like, a temperature condition for thevicinity of the nozzle is set up strictly into a range of not causingdevitrification in the glass. The molten glass flowing out is cast intoa receiving die having a receiving surface of planar shape, concaveshape, convex shape or the like or into a molding die in which anenclosure is provided in the circumference of a planar surface, aconcave surface or a convex surface. As a result, the molten glass isformed into a desired shape. In the following description, preferableforming methods are illustrated.

A first forming method is one example of producing methods for the gobpreform. First, a molten glass lump having a weight corresponding to afinal molded article or alternatively a desired weight including anaddition necessary for secondary processing into a final molded articleis dropped into each of a plurality of receiving dies arranged under thenozzle. Then, cooling is performed simultaneously to the forming ofglass lumps so that gob preforms are obtained.

A second forming method is another example of producing methods for thegob preform. The second forming method is suitable for a case that apreform having a relatively heavy weight is to be produced. First, thetip of the molten glass flowing out through the nozzle is brought intocontact with the receiving die surface. Then, at each time when adesired weight is reached, the receiving die is rapidly separated fromthe molten glass so that the molten glass is cut off. As a result, theforming of glass free from striae and shear marks is achieved. Further,when necessary, press molding may be performed with aimingsimultaneously at cooling of the molten glass lump, so that a desiredshape may be imparted and gob preforms are obtained.

A third forming method is one example of producing methods for thepolished preform. First, a glass lump having a desired shape is producedby a similar method to the first and the second forming methods. At thistime, the glass lump having a desired shape has a total weight of adesired weight plus an additional weight necessary for finishing all thesurfaces including optically functional surfaces of a final product(such as a lens) by means of machining. Then, the glass lump having adesired shape is cut and polished by means of machining so that polishedpreforms are obtained.

The preform obtained by each of the first and the second forming methodsdescribed above can be used as a preform for press molding directly inprecision press molding. Further, the third forming method provides apreform for polish. In order that breakage should be avoided in thepreform at the time of handling, for example, a three-dimensionalcooling method, an optimal cooling rate, an annealing treatment and thelike may be selected depending on the shape and the weight.

As described above, a preform for press molding and a preform for polishhaving a desired weight can be obtained from the optical glasscomposition according to Embodiment III-1. In the case of a preform forpress molding, for the purpose of mold release at the time of forming,it is preferable that the surface roughness of the receiving die surfaceis adjusted or alternatively that a mold-release film is formed. In thecase of a preform for polish, when an HBN (boron-containingmold-releasing agent) is applied, releasing from the die becomes easier.Thus, this approach is preferable.

(Embodiment III-3)

Next, an optical element according to Embodiment III-3 of the presentinvention is described below. The optical element according toEmbodiment III-3 has optical indices determined by the composition ofthe optical glass composition according to the above-mentionedEmbodiment III-1. That is, the refractive index (nd) to the d-line is1.83 to 1.87, while the Abbe number (vd) to the d-line is 43 to 47, andwhile the liquidus temperature is 1300° C. or lower. Further, thisoptical element has also a feature that optical absorption by coloringis low in a visible light range. The optical element employing theoptical glass composition according to Embodiment III-1 is an opticalelement suitable for an optical system in a digital camera, a videocamera, a mobile device or the like.

Examples of the optical element according to the present EmbodimentIII-3 include a spherical lens, an aspheric lens and a micro lens, aswell as a prism and a diffraction grating. Other examples include anoptical element cemented with an optical element composed of a glassmaterial or an optical material of another kind.

Next, a producing method for the optical element according to EmbodimentIII-3 is described below. The optical element according to EmbodimentIII-3 can be produced by supplying the preform according to theabove-mentioned Embodiment III-2 into a molding die, then softening itby heating, then performing press molding, and then performing polishwhen necessary.

Press molding methods employable for obtaining the optical element aredivided into two typical methods. These methods are selected dependingon the means of forming optically functional surfaces where light entersand exits.

First means is means referred to as precision press molding. The moldingsurfaces of a press molding die are precisely processed in advance intoreversal shapes of the optically functional surfaces of the opticalelement serving as a final molded article. Then, when necessary, amold-release film is provided in order to avoid fusion between thepreform and the molding die. Then, press molding is performed so thatthe shape of the above-mentioned molding surface is preciselytransferred to the preform for press molding having been softened byheating. According to this means, grinding and polishing of opticallyfunctional surfaces are unnecessary. That is, the optical element can beproduced solely by press molding. The press molding is performed in aninert atmosphere like in nitrogen gas. Here, when a preform for pressmolding having an additional weight relative to the final molded articleis used, for example, in the case of a lens, centering may be performedby grinding by the amount corresponding to the additional weight.

Second means is that press molding is performed using a preform forpolish having a shape which is similar to the shape of the opticalelement serving as a final molded article and which is larger than theoptical element. The formed press molded article contains opticallyfunctional surfaces, and the surfaces of the optical element are formedby machining. In the press molded article, in order that breakage thatcould be caused by the machining should be avoided in the glass,residual strain need be minimized. Further, also in order that requiredoptical indices should be achieved, an appropriate annealing treatmentis necessary. According to this means, press molding may be performed inan ordinary atmosphere. Further, the above-mentioned mold-releasingagent may be used.

Here, in both cases regardless of whether the first means or the secondmeans described above is selected, the refractive index (nd) and theAbbe number (vd) of the obtained optical element varies slightly owingto the heat history in the producing process. Thus, when an opticalelement having precisely specified optical indices is to be produced,component adjustment for the optical glass composition, heat historyadjustment in the producing process, adjustment of incorporating theamount of variation into the optical design when necessary, or the likemay be selected appropriately with taking into consideration theabove-mentioned variation in the refractive index (nd) and the Abbenumber (vd). As a result, an optical element is obtained that hasdesired optical indices and an excellent transmissivity and that isparticularly suitable as an optical component of a device whichincorporates a solid-state image sensor or the like.

(Embodiment IV-1)

First, an optical glass composition according to Embodiment IV-1 of thepresent invention is described below in detail. This optical glasscomposition has the following composition.

That is, the optical glass composition according to the presentEmbodiment IV-1 contains, in % by mole, 0% or more and 10.0% or less ofSiO₂, 30.0% or more and 45.0% or less of B₂O₃, 0% or more and 5.0% orless of Li₂O, 0% or more and 12.0% or less of ZnO, 0% or more and 10.0%or less of ZrO₂, 10.0% or more and 20.0% or less of La₂O₃, 3.0% or moreand 10.0% or less of Ta₂O₅, 11.0% or more and 20.0% or less ofTa₂O₅+ZrO₂ and 5.0% or more and 20.0% or less of Gd₂O₃. From thisoptical glass composition, more stable high refractive index-lower tomiddle dispersion type optical glass is obtained that has a refractiveindex (nd) to the d-line of 1.83 or higher and 1.86 or lower and an Abbenumber (vd) to the d-line of 43 or higher and 45 or lower, and yet has aliquidus temperature of 1200° C. or lower.

Next, the individual components contained in the optical glasscomposition are described below in detail. Hereinafter, the contents ofthe individual components are expressed in % by mole.

SiO₂ serves as a component for composing a network, and is a componentfor improving devitrification resistance. Nevertheless, when anexcessive amount of SiO₂ is used, its solubility becomes poor, and hencedifficulty arises in stable preparing. Thus, the amount of SiO₂ is setto be 0% or more and 10.0% or less and, preferably, 0% or more and 9.5%or less. Here, in order to prevent the devitrification resistance frombecoming poor and the glass from becoming unstable, it is preferablethat the amount of SiO₂ is 7.0% or more.

B₂O₃ serves as a component for composing a network, and has the effectof lowering a temperature range necessary for ensuring a desired meltingproperty and a desired viscous flow. Nevertheless, when the amount ofB₂O₃ exceeds 45.0%, the refractive index becomes excessively low. Incontrast, when the amount of B₂O₃ is less than 30.0%, the temperaturerange necessary for ensuring a desired melting property and a desiredfluidity becomes excessively high. A preferable amount of B₂O₃ is 40.0%or more and 44.0% or less.

Li₂O has the effect of lowering Tg so as to improve the meltingproperty. Nevertheless, when an excessive amount of Li₂O is used,remarkable degradation arises in the devitrification resistance and therefractive index. Thus, the amount of Li₂O is set to be 0% or more and5.0% or less and, preferably, 0% or more and 3.0% or less. Here, inorder that Tg should be lowered so that the effect of improving themelting property should be achieved more successfully, it is preferablethat the amount of Li₂O is 0.5% or more.

Similarly to Li₂O, K₂O and Na₂O have the effect of lowering Tg so as toimprove the melting property. Nevertheless, the use of K₂O and Na₂Ocauses a possibility that remarkable degradation of the devitrificationresistance and the refractive index is accelerated. Thus, when K₂O andNa₂O need be used, the amount of each is set to be 0% or more and 6.0%or less.

ZnO has the effects of improving the devitrification resistance andlowering the temperature necessary for viscous flow. Nevertheless, whenan excessive amount of ZnO is used, difficulty arises in adjusting therefractive index (nd) and the Abbe number (vd) into desired ranges. Theamount of ZnO is 0% or more and 12.0% or less. Here, in order that theeffects of improving the devitrification resistance and lowering thetemperature necessary for viscous flow should be achieved moresuccessfully, it is preferable that the amount of ZnO is 1.0% or more.

ZrO₂ has the effects of improving the refractive index and alsoimproving the devitrification resistance. Nevertheless, when anexcessive amount of ZrO₂ is used, the devitrification resistance becomespoor and so does the solubility. The amount of ZrO₂ is 0% or more and10.0% or less. Here, in order that the effects of improving therefractive index and also improving the devitrification resistanceshould be achieved more successfully, it is preferable that the amountof ZrO₂ is 1.0% or more.

La₂O₃ improves the refractive index, and is one of the most importantcomponents that control the Abbe number. When the amount of La₂O₃ isless than 10.0%, difficulty arises in adjusting the Abbe number into adesired range. In contrast, when the amount of La₂O₃ exceeds 20.0%, thedevitrification resistance becomes poor. Thus, glass becomes unstable,and hence difficulty arises in preparing. A preferable amount of La₂O₃is 11.0% or more and 19.0% or less.

Similarly to La₂O₃, Ta₂O₅ improves the refractive index, and is one ofthe components that control the Abbe number. Also, Ta₂O₅ is one of thecomponents that decrease the liquidus temperature to a desired range.Nevertheless, when the amount of Ta₂O₅ exceeds 10.0%, the meltingproperty becomes poor, and hence difficulty arises in preparing. Incontrast, when the amount of Ta₂O₅ is less than 3.0%, the effects ofimproving the refractive index and decreasing the liquidus temperaturebecome insufficient. A preferable amount of Ta₂O₅ is 5.0% or more and7.0% or less.

Here, in order that the refractive index should be increased to adesired range and the liquidus temperature should be decreased to adesired range, the total amount of Ta₂O₅ and ZrO₂ (Ta₂O₅+ZrO₂) isadjusted into 11.0% or more. Here, when the total amount is excessive,the devitrification resistance becomes poor. Thus, the total amount isadjusted into 20.0% or less, preferably, into 15.0% or less.

Similarly to La₂O₃, Gd₂O₃ improves the refractive index, and is one ofthe components that control the Abbe number. Nevertheless, when theamount of Gd₂O₃ exceeds 20.0%, the devitrification resistance becomespoor. Thus, glass becomes unstable, and hence difficulty arises inpreparing. In contrast, the amount of Gd₂O₃ is less than 5.0%, theeffect of improving the refractive index becomes insufficient. Apreferable amount of Gd₂O₃ is 8.0% or more and 17.0% or less.

GeO₂ may be used as a replacement of SiO₂, and serves as a component forcomposing a network. Nevertheless, when an excessive amount of GeO₂ isused, this causes a possibility that the devitrification resistancebecomes poor. Thus, it is preferable that the amount of GeO₂ is 0% ormore and 16.0% or less and, more preferably, 0% or more and 8.0% orless.

BaO is a component that improves preparing property, and may be usedwithin a range of 0% or more and 10.0% or less. Here, alkaline earthmetal oxides R′O (here, R′ is at least one of Sr, Ca and Mg) other thanthe BaO have a tendency that when an excessive amount is used, thedevitrification resistance becomes poor. Thus, non-use of these ispreferable. Accordingly, when the use of R′O is unavoidable, it ispreferable that their total amount is set to be 15.0% or less.

Similarly to La₂O₃, Nb₂O₅ improves the refractive index, and is one ofthe components that control the Abbe number. Further, when La₂O₃ isreplaced by Nb₂O₅, the effect of improving the devitrificationresistance is also obtained. Nevertheless, when an excessive amount ofNb₂O₅ is used, difficulty arises in adjusting the Abbe number into adesired range. Thus, the amount of Nb₂O₅ is set to be 0% or more and3.0% or less and, preferably, 0% or more and 2.0% or less. Here, inorder that the effect of improving the refractive index and thedevitrification resistance should be achieved more successfully, it ispreferable that the amount of Nb₂O₅ is 0.5% or more.

TiO₂ has the effects of controlling the refractive index and the Abbenumber and improving the devitrification resistance. Nevertheless, whenan excessive amount of TiO₂ is used, difficulty arises in adjusting theAbbe number into a desired range. Thus, the amount of TiO₂ is set to be0% or more and 3.0% or less and, preferably, 0% or more and 2.0% orless. Here, in order that the effect of improving the refractive indexand the devitrification resistance should be achieved more successfully,it is preferable that the amount of TiO₂ is 0.5% or more.

Similarly to La₂O₃, Y₂O₃ and Yb₂O₃ improve the refractive index, and arecomponents that control the Abbe number. Nevertheless, when an excessiveamount of these Y₂O₃ and Yb₂O₃ is used, the devitrification resistancebecomes poor. Thus, glass becomes unstable, and hence difficulty arisesin preparing. Thus, when La₂O₃ need be replaced by these Y₂O₃ and Yb₂O₃,it is preferable that the amount of each is set to be 0% or more and3.0% or less.

WO₃ is a component for alleviating the high devitrification caused byLa₂O₃ and adjusting the refractive index and the Abbe number intodesired ranges. Nevertheless, when an excessive amount of WO₃ is used,this causes a possibility that the transmissivity in a blue light rangebecomes poor. Thus, the amount of WO₃ is 0% or more and 3.0% or lessand, preferably, 0% or more and 2.0% or less. Here, in order that theeffects of alleviating the high devitrification caused by La₂O₃ andadjusting the refractive index and the Abbe number into desired rangesshould be achieved more successfully, it is preferable that the amountof WO₃ is 0.5% or more.

Al₂O₃ may be used for adjusting the refractive index. However, it ispreferable that the amount is 0% or more and 10% or less. Further, inorder to adjust the refractive index, Ga₂O₃ and In₂O₃ may be used in anamount up to approximately 10% each. Nevertheless, the use of thesecauses a possibility that the devitrification resistance becomes poor.Thus, it is preferable that these are not used in an excessive amount.

In addition to the above-mentioned components, Sb₂O₃ and SnO₂ may beused which are generally used as fining agents. Here, it is preferablethat the amount of Sb₂O₃ and SnO₂ is 0% or more and 2% or less each.Nevertheless, As₂O₃ having a strong effect as a fining agent is toxic.Thus, it is preferable that As₂O₃ is not used.

In addition, as for Pb and its compounds, compounds including Te, Se, orCd, as well as radioactive substances such as compounds including U orTh, it is preferable that they are not used from the viewpoint ofsafety. Further, it is also preferable that substances such as compoundsincluding Cu, Cr, V, Fe, Ni or Co that cause coloring are not used.

When the individual components are adjusted into the above-mentionedratios, a high refractive index-lower to middle dispersion type opticalglass composition is obtained that has a refractive index (nd) to thed-line of 1.83 or higher and 1.86 or lower and an Abbe number (vd) tothe d-line of 43 or higher and 45 or lower, and yet has a liquidustemperature of 1200° C. or lower.

Here, when the glass is handled in a softened or molten state, from theviewpoints of melting property, processability (droplet property),crystallinity and the like, a lower liquidus temperature is generallypreferable in the optical glass. Accordingly, as mentioned above, theoptical glass composition according to the present Embodiment IV-1 has aliquidus temperature of 1200° C. or lower and, preferably, 1190° C. orlower.

(Embodiment IV-2)

Next, a preform according to Embodiment IV-2 of the present inventionand a producing method for the same are described below in detail. Thepreform is softened by heating so as to be used at least for pressmolding. Thus, the weight and the shape of the preform are determinedappropriately in accordance with the size and the shape of a pressmolded article serving as a final target. The preform according to thepresent Embodiment IV-2 is formed from the optical glass compositionaccording to the above-mentioned Embodiment IV-1, and hence obtainedwithout losing various features of the optical glass composition inEmbodiment IV-1.

The preform produced from the optical glass composition according toEmbodiment IV-1 is described below. Here, the preform is a preliminaryglass form that is heated and used for precision press molding. Thepreforms are divided into a gob preform produced by forming a moltenglass material and a polished preform produced by physically polishing aglass material. The optical glass composition according to EmbodimentIV-1 can be used for both of the gob preform and the polished preform.

The producing method for the preform is described below. First, glassraw materials (the above-mentioned individual components) for theoptical glass composition according to the above-mentioned EmbodimentIV-1 are weighed and prepared, and then processed in processing stepssuch as dissolution, defoaming, fining and homogenization so thathomogeneous molten glass is obtained which contains no foreignsubstances therein. Then, at the time when the molten glass is caused toflow out through an outflow pipe (referred to as a nozzle, hereinafter)made of platinum alloy or the like, a temperature condition for thevicinity of the nozzle is set up strictly into a range of not causingdevitrification in the glass. The molten glass flowing out is cast intoa receiving die having a receiving surface of planar shape, concaveshape, convex shape or the like or into a molding die in which anenclosure is provided in the circumference of a planar surface, aconcave surface or a convex surface. As a result, the molten glass isformed into a desired shape. In the following description, preferableforming methods are illustrated.

A first forming method is one example of producing methods for the gobpreform. First, a molten glass lump having a weight corresponding to afinal molded article or alternatively a desired weight including anaddition necessary for secondary processing into a final molded articleis dropped into each of a plurality of receiving dies arranged under thenozzle. Then, cooling is performed simultaneously to the forming ofglass lumps so that gob preforms are obtained.

A second forming method is another example of producing methods for thegob preform. The second forming method is suitable for a case that apreform having a relatively heavy weight is to be produced. First, thetip of the molten glass flowing out through the nozzle is brought intocontact with the receiving die surface. Then, at each time when adesired weight is reached, the receiving die is rapidly separated fromthe molten glass so that the molten glass is cut off. As a result, theforming of glass free from striae and shear marks is achieved. Further,when necessary, press molding may be performed with aimingsimultaneously at cooling of the molten glass lump, so that a desiredshape may be imparted and gob preforms are obtained.

A third forming method is one example of producing methods for thepolished preform. First, a glass lump having a desired shape is producedby a similar method to the first and the second forming methods. At thistime, the glass lump having a desired shape has a total weight of adesired weight plus an additional weight necessary for finishing all thesurfaces including optically functional surfaces of a final product(such as a lens) by means of machining. Then, the glass lump having adesired shape is cut and polished by means of machining so that polishedpreforms are obtained.

The preform obtained by each of the first and the second forming methodsdescribed above can be used as a preform for press molding directly inprecision press molding. Further, the third forming method provides apreform for polish. In order that breakage should be avoided in thepreform at the time of handling, for example, a three-dimensionalcooling method, an optimal cooling rate, an annealing treatment and thelike may be selected depending on the shape and the weight.

As described above, a preform for press molding and a preform for polishhaving a desired weight can be obtained from the optical glasscomposition according to Embodiment IV-1. In the case of a preform forpress molding, for the purpose of mold release at the time of forming,it is preferable that the surface roughness of the receiving die surfaceis adjusted or alternatively that a mold-release film is formed. In thecase of a preform for polish, when an HBN (boron-containingmold-releasing agent) is applied, releasing from the die becomes easier.Thus, this approach is preferable.

(Embodiment IV-3)

Next, an optical element according to Embodiment IV-3 of the presentinvention is described below. The optical element according toEmbodiment IV-3 has optical indices determined by the composition of theoptical glass composition according to the above-mentioned EmbodimentIV-1. That is, the refractive index (nd) to the d-line is 1.83 to 1.86,while the Abbe number (vd) to the d-line is 43 to 45, and while theliquidus temperature is 1200° C. or lower. Further, this optical elementhas also a feature that optical absorption by coloring is low in avisible light range. The optical element employing the optical glasscomposition according to Embodiment IV-1 is an optical element suitablefor an optical system in a digital camera, a video camera, a mobiledevice or the like.

Examples of the optical element according to the present Embodiment IV-3include a spherical lens, an aspheric lens and a micro lens, as well asa prism and a diffraction grating. Other examples include an opticalelement cemented with an optical element composed of a glass material oran optical material of another kind.

Next, a producing method for the optical element according to EmbodimentIV-3 is described below. The optical element according to EmbodimentIV-3 can be produced by supplying the preform according to theabove-mentioned Embodiment W-2 into a molding die, then softening it byheating, then performing press molding, and then performing polish whennecessary.

Press molding methods employable for obtaining the optical element aredivided into two typical methods. These methods are selected dependingon the means of forming optically functional surfaces where light entersand exits.

First means is means referred to as precision press molding. The moldingsurfaces of a press molding die are precisely processed in advance intoreversal shapes of the optically functional surfaces of the opticalelement serving as a final molded article. Then, when necessary, amold-release film is provided in order to avoid fusion between thepreform and the molding die. Then, press molding is performed so thatthe shape of the above-mentioned molding surface is preciselytransferred to the preform for press molding having been softened byheating. According to this means, grinding and polishing of opticallyfunctional surfaces are unnecessary. That is, the optical element can beproduced solely by press molding. The press molding is performed in aninert atmosphere like in nitrogen gas. Here, when a preform for pressmolding having an additional weight relative to the final molded articleis used, for example, in the case of a lens, centering may be performedby grinding by the amount corresponding to the additional weight.

Second means is that press molding is performed using a preform forpolish having a shape which is similar to the shape of the opticalelement serving as a final molded article and which is larger than theoptical element. The formed press molded article contains opticallyfunctional surfaces, and the surfaces of the optical element are formedby machining. In the press molded article, in order that breakage thatcould be caused by the machining should be avoided in the glass,residual strain need be minimized. Further, also in order that requiredoptical indices should be achieved, an appropriate annealing treatmentis necessary. According to this means, press molding may be performed inan ordinary atmosphere. Further, the above-mentioned mold-releasingagent may be used.

Here, in both cases regardless of whether the first means or the secondmeans described above is selected, the refractive index (nd) and theAbbe number (vd) of the obtained optical element varies slightly owingto the heat history in the producing process. Thus, when an opticalelement having precisely specified optical indices is to be produced,component adjustment for the optical glass composition, heat historyadjustment in the producing process, adjustment of incorporating theamount of variation into the optical design when necessary, or the likemay be selected appropriately with taking into consideration theabove-mentioned variation in the refractive index (nd) and the Abbenumber (vd). As a result, an optical element is obtained that hasdesired optical indices and an excellent transmissivity and that isparticularly suitable as an optical component of a device whichincorporates a solid-state image sensor or the like.

(Embodiment V-1)

First, an optical glass composition according to Embodiment V-1 of thepresent invention is described below in detail. This optical glasscomposition has the following composition.

That is, the optical glass composition according to the presentEmbodiment V-1 contains, in % by mole, 0% or more and 10.0% or less ofSiO₂, 35.0% or more and 45.0% or less of B₂O₃, 0% or more and 5.0% orless of Li₂O, 0% or more and 12.0% or less of ZnO, 0% or more and 10.0%or less of ZrO₂, 10.0% or more and 20.0% or less of La₂O₃, 3.0% or moreand 10.0% or less of Ta₂O₅, 10.0% or more and 22.0% or less of Ta₂O₅+ZnOand 5.0% or more and 20.0% or less of Gd₂O₃. From this optical glasscomposition, more stable high refractive index-lower to middledispersion type optical glass is obtained that has a refractive index(nd) to the d-line of 1.83 or higher and 1.86 or lower and an Abbenumber (vd) to the d-line of 43 or higher and 46 or lower, and yet has aliquidus temperature of 1200° C. or lower.

Next, the individual components contained in the optical glasscomposition are described below in detail. Hereinafter, the contents ofthe individual components are expressed in % by mole.

SiO₂ serves as a component for composing a network, and is a componentfor improving devitrification resistance. Nevertheless, when anexcessive amount of SiO₂ is used, its solubility becomes poor, and hencedifficulty arises in stable preparing. Thus, the amount of SiO₂ is setto be 0% or more and 10.0% or less and, preferably, 0% or more and 9.5%or less. Here, in order to prevent the devitrification resistance frombecoming poor and the glass from becoming unstable, it is preferablethat the amount of SiO₂ is 7.0% or more.

B₂O₃ serves as a component for composing a network, and has the effectof lowering a temperature range necessary for ensuring a desired meltingproperty and a desired viscous flow. Nevertheless, when the amount ofB₂O₃ exceeds 45.0%, the refractive index becomes excessively low. Incontrast, when the amount of B₂O₃ is less than 35.0%, the temperaturerange necessary for ensuring a desired melting property and a desiredfluidity becomes excessively high. A preferable amount of B₂O₃ is 40.0%or more and 44.0% or less.

Li₂O has the effect of lowering Tg so as to improve the meltingproperty. Nevertheless, when an excessive amount of Li₂O is used,remarkable degradation arises in the devitrification resistance and therefractive index. Thus, the amount of Li₂O is set to be 0% or more and5.0% or less and, preferably, 0% or more and 4.0% or less. Here, inorder that Tg should be lowered so that the effect of improving themelting property should be achieved more successfully, it is preferablethat the amount of Li₂O is 0.5% or more.

Similarly to Li₂O, K₂O and Na₂O have the effect of lowering Tg so as toimprove the melting property. Nevertheless, the use of K₂O and Na₂Ocauses a possibility that remarkable degradation of the devitrificationresistance and the refractive index is accelerated. Thus, when K₂O andNa₂O need be used, the amount of each is set to be 0% or more and 6.0%or less.

ZnO has the effects of improving the devitrification resistance andlowering the temperature necessary for viscous flow. Nevertheless, whenan excessive amount of ZnO is used, difficulty arises in adjusting therefractive index (nd) and the Abbe number (vd) into desired ranges. Theamount of ZnO is 0% or more and 12.0% or less. Here, in order that theeffects of improving the devitrification resistance and lowering thetemperature necessary for viscous flow should be achieved moresuccessfully, it is preferable that the amount of ZnO is 1.0% or more.

ZrO₂ has the effects of improving the refractive index and alsoimproving the devitrification resistance. Nevertheless, when anexcessive amount of ZrO₂ is used, the devitrification resistance becomespoor and so does the solubility. The amount of ZrO₂ is 0% or more and10.0% or less. Here, in order that the effects of improving therefractive index and also improving the devitrification resistanceshould be achieved more successfully, it is preferable that the amountof ZrO₂ is 1.0% or more.

La₂O₃ improves the refractive index, and is one of the most importantcomponents that control the Abbe number. When the amount of La₂O₃ isless than 10.0%, difficulty arises in adjusting the Abbe number into adesired range. In contrast, when the amount of La₂O₃ exceeds 20.0%, thedevitrification resistance becomes poor. Thus, glass becomes unstable,and hence difficulty arises in preparing. A preferable amount of La₂O₃is 11.0% or more and 19.0% or less.

Similarly to La₂O₃, Ta₂O₅ improves the refractive index, and is one ofthe components that control the Abbe number. Also, Ta₂O₅ is one of thecomponents that decrease the liquidus temperature to a desired range.Nevertheless, when the amount of Ta₂O₅ exceeds 10.0%, the meltingproperty becomes poor, and hence difficulty arises in preparing. Incontrast, when the amount of Ta₂O₅ is less than 3.0%, the effects ofimproving the refractive index and decreasing the liquidus temperaturebecome insufficient. A preferable amount of Ta₂O₅ is 5.0% or more and7.0% or less.

Here, in order that the refractive index should be increased to adesired range and the liquidus temperature should be decreased to adesired range, the total amount of Ta₂O₅ and ZnO(Ta₂O₅+ZnO) is adjustedinto 10.0% or more. Here, when the total amount is excessive, thedevitrification resistance becomes poor. Thus, the total amount isadjusted into 22.0% or less, preferably, into 18.0% or less.

Similarly to La₂O₃, Gd₂O₃ improves the refractive index, and is one ofthe components that control the Abbe number. Nevertheless, when theamount of Gd₂O₃ exceeds 20.0%, the devitrification resistance becomespoor. Thus, glass becomes unstable, and hence difficulty arises inpreparing. In contrast, the amount of Gd₂O₃ is less than 5.0%, theeffect of improving the refractive index becomes insufficient. Apreferable amount of Gd₂O₃ is 8.0% or more and 17.0% or less.

GeO₂ may be used as a replacement of SiO₂, and serves as a component forcomposing a network. Nevertheless, when an excessive amount of GeO₂ isused, this causes a possibility that the devitrification resistancebecomes poor. Thus, it is preferable that the amount of GeO₂ is 0% ormore and 16.0% or less and, more preferably, 0% or more and 8.0% orless.

BaO is a component that improves preparing property, and may be usedwithin a range of 0% or more and 10.0% or less. Here, alkaline earthmetal oxides R′O (here, R′ is at least one of Sr, Ca and Mg) other thanthe BaO have a tendency that when an excessive amount is used, thedevitrification resistance becomes poor. Thus, non-use of these ispreferable. Accordingly, when the use of R′O is unavoidable, it ispreferable that their total amount is set to be 15.0% or less.

Similarly to La₂O₃, Nb₂O₅ improves the refractive index, and is one ofthe components that control the Abbe number. Further, when La₂O₃ isreplaced by Nb₂O₅, the effect of improving the devitrificationresistance is also obtained. Nevertheless, when an excessive amount ofNb₂O₅ is used, difficulty arises in adjusting the Abbe number into adesired range. Thus, the amount of Nb₂O₅ is set to be 0% or more and3.0% or less and, preferably, 0% or more and 2.0% or less. Here, inorder that the effect of improving the refractive index and thedevitrification resistance should be achieved more successfully, it ispreferable that the amount of Nb₂O₅ is 0.5% or more.

TiO₂ has the effects of controlling the refractive index and the Abbenumber and improving the devitrification resistance. Nevertheless, whenan excessive amount of TiO₂ is used, difficulty arises in adjusting theAbbe number into a desired range. Thus, the amount of TiO₂ is set to be0% or more and 3.0% or less and, preferably, 0% or more and 2.0% orless. Here, in order that the effect of improving the refractive indexand the devitrification resistance should be achieved more successfully,it is preferable that the amount of TiO₂ is 0.5% or more.

Similarly to La₂O₃, Y₂O₃ and Yb₂O₃ improve the refractive index, and arecomponents that control the Abbe number. Nevertheless, when an excessiveamount of these Y₂O₃ and Yb₂O₃ is used, the devitrification resistancebecomes poor. Thus, glass becomes unstable, and hence difficulty arisesin preparing. Thus, when La₂O₃ need be replaced by these Y₂O₃ and Yb₂O₃,it is preferable that the amount of each is set to be 0% or more and3.0% or less.

WO₃ is a component for alleviating the high devitrification caused byLa₂O₃ and adjusting the refractive index and the Abbe number intodesired ranges. Nevertheless, when an excessive amount of WO₃ is used,this causes a possibility that the transmissivity in a blue light rangebecomes poor. Thus, the amount of WO₃ is 0% or more and 3.0% or lessand, preferably, 0% or more and 2.0% or less. Here, in order that theeffects of alleviating the high devitrification caused by La₂O₃ andadjusting the refractive index and the Abbe number into desired rangesshould be achieved more successfully, it is preferable that the amountof WO₃ is 0.5% or more.

Al₂O₃ may be used for adjusting the refractive index. However, it ispreferable that the amount is 0% or more and 10% or less. Further, inorder to adjust the refractive index, Ga₂O₃ and In₂O₃ may be used in anamount up to approximately 10% each. Nevertheless, the use of thesecauses a possibility that the devitrification resistance becomes poor.Thus, it is preferable that these are not used in an excessive amount.

In addition to the above-mentioned components, Sb₂O₃ and SnO₂ may beused which are generally used as fining agents. Here, it is preferablethat the amount of Sb₂O₃ and SnO₂ is 0% or more and 2% or less each.Nevertheless, As₂O₃ having a strong effect as a fining agent is toxic.Thus, it is preferable that As₂O₃ is not used.

In addition, as for Pb and its compounds, compounds including Te, Se, orCd, as well as radioactive substances such as compounds including U orTh, it is preferable that they are not used from the viewpoint ofsafety. Further, it is also preferable that substances such as compoundsincluding Cu, Cr, V, Fe, Ni or Co that cause coloring are not used.

When the individual components are adjusted into the above-mentionedratios, a high refractive index-lower to middle dispersion type opticalglass composition is obtained that has a refractive index (nd) to thed-line of 1.83 or higher and 1.86 or lower and an Abbe number (vd) tothe d-line of 43 or higher and 46 or lower, and yet has a liquidustemperature of 1200° C. or lower.

Here, when the glass is handled in a softened or molten state, from theviewpoints of melting property, processability (droplet property),crystallinity and the like, a lower liquidus temperature is generallypreferable in the optical glass. Accordingly, as mentioned above, theoptical glass composition according to the present Embodiment V-1 has aliquidus temperature of 1200° C. or lower and, preferably, 1190° C. orlower.

(Embodiment V-2)

Next, a preform according to Embodiment V-2 of the present invention anda producing method for the same are described below in detail. Thepreform is softened by heating so as to be used at least for pressmolding. Thus, the weight and the shape of the preform are determinedappropriately in accordance with the size and the shape of a pressmolded article serving as a final target. The preform according to thepresent Embodiment V-2 is formed from the optical glass compositionaccording to the above-mentioned Embodiment V-1, and hence obtainedwithout losing various features of the optical glass composition inEmbodiment V-1.

The preform produced from the optical glass composition according toEmbodiment V-1 is described below. Here, the preform is a preliminaryglass form that is heated and used for precision press molding. Thepreforms are divided into a gob preform produced by forming a moltenglass material and a polished preform produced by physically polishing aglass material. The optical glass composition according to EmbodimentV-1 can be used for both of the gob preform and the polished preform.

The producing method for the preform is described below. First, glassraw materials (the above-mentioned individual components) for theoptical glass composition according to the above-mentioned EmbodimentV-1 are weighed and prepared, and then processed in processing stepssuch as dissolution, defoaming, fining and homogenization so thathomogeneous molten glass is obtained which contains no foreignsubstances therein. Then, at the time when the molten glass is caused toflow out through an outflow pipe (referred to as a nozzle, hereinafter)made of platinum alloy or the like, a temperature condition for thevicinity of the nozzle is set up strictly into a range of not causingdevitrification in the glass. The molten glass flowing out is cast intoa receiving die having a receiving surface of planar shape, concaveshape, convex shape or the like or into a molding die in which anenclosure is provided in the circumference of a planar surface, aconcave surface or a convex surface. As a result, the molten glass isformed into a desired shape. In the following description, preferableforming methods are illustrated.

A first forming method is one example of producing methods for the gobpreform. First, a molten glass lump having a weight corresponding to afinal molded article or alternatively a desired weight including anaddition necessary for secondary processing into a final molded articleis dropped into each of a plurality of receiving dies arranged under thenozzle. Then, cooling is performed simultaneously to the forming ofglass lumps so that gob preforms are obtained.

A second forming method is another example of producing methods for thegob preform. The second forming method is suitable for a case that apreform having a relatively heavy weight is to be produced. First, thetip of the molten glass flowing out through the nozzle is brought intocontact with the receiving die surface. Then, at each time when adesired weight is reached, the receiving die is rapidly separated fromthe molten glass so that the molten glass is cut off. As a result, theforming of glass free from striae and shear marks is achieved. Further,when necessary, press molding may be performed with aimingsimultaneously at cooling of the molten glass lump, so that a desiredshape may be imparted and gob preforms are obtained.

A third forming method is one example of producing methods for thepolished preform. First, a glass lump having a desired shape is producedby a similar method to the first and the second forming methods. At thistime, the glass lump having a desired shape has a total weight of adesired weight plus an additional weight necessary for finishing all thesurfaces including optically functional surfaces of a final product(such as a lens) by means of machining. Then, the glass lump having adesired shape is cut and polished by means of machining so that polishedpreforms are obtained.

The preform obtained by each of the first and the second forming methodsdescribed above can be used as a preform for press molding directly inprecision press molding. Further, the third forming method provides apreform for polish. In order that breakage should be avoided in thepreform at the time of handling, for example, a three-dimensionalcooling method, an optimal cooling rate, an annealing treatment and thelike may be selected depending on the shape and the weight.

As described above, a preform for press molding and a preform for polishhaving a desired weight can be obtained from the optical glasscomposition according to Embodiment V-1. In the case of a preform forpress molding, for the purpose of mold release at the time of forming,it is preferable that the surface roughness of the receiving die surfaceis adjusted or alternatively that a mold-release film is formed. In thecase of a preform for polish, when an HBN (boron-containingmold-releasing agent) is applied, releasing from the die becomes easier.Thus, this approach is preferable.

(Embodiment V-3)

Next, an optical element according to Embodiment V-3 of the presentinvention is described below. The optical element according toEmbodiment V-3 has optical indices determined by the composition of theoptical glass composition according to the above-mentioned EmbodimentV-1. That is, the refractive index (nd) to the d-line is 1.83 to 1.86,while the Abbe number (vd) to the d-line is 43 to 46, and while theliquidus temperature is 1200° C. or lower. Further, this optical elementhas also a feature that optical absorption by coloring is low in avisible light range. The optical element employing the optical glasscomposition according to Embodiment V-1 is an optical element suitablefor an optical system in a digital camera, a video camera, a mobiledevice or the like.

Examples of the optical element according to the present Embodiment V-3include a spherical lens, an aspheric lens and a micro lens, as well asa prism and a diffraction grating. Other examples include an opticalelement cemented with an optical element composed of a glass material oran optical material of another kind.

Next, a producing method for the optical element according to EmbodimentV-3 is described below. The optical element according to Embodiment V-3can be produced by supplying the preform according to theabove-mentioned Embodiment V-2 into a molding die, then softening it byheating, then performing press molding, and then performing polish whennecessary.

Press molding methods employable for obtaining the optical element aredivided into two typical methods. These methods are selected dependingon the means of forming optically functional surfaces where light entersand exits.

First means is means referred to as precision press molding. The moldingsurfaces of a press molding die are precisely processed in advance intoreversal shapes of the optically functional surfaces of the opticalelement serving as a final molded article. Then, when necessary, amold-release film is provided in order to avoid fusion between thepreform and the molding die. Then, press molding is performed so thatthe shape of the above-mentioned molding surface is preciselytransferred to the preform for press molding having been softened byheating. According to this means, grinding and polishing of opticallyfunctional surfaces are unnecessary. That is, the optical element can beproduced solely by press molding. The press molding is performed in aninert atmosphere like in nitrogen gas. Here, when a preform for pressmolding having an additional weight relative to the final molded articleis used, for example, in the case of a lens, centering may be performedby grinding by the amount corresponding to the additional weight.

Second means is that press molding is performed using a preform forpolish having a shape which is similar to the shape of the opticalelement serving as a final molded article and which is larger than theoptical element. The formed press molded article contains opticallyfunctional surfaces, and the surfaces of the optical element are formedby machining. In the press molded article, in order that breakage thatcould be caused by the machining should be avoided in the glass,residual strain need be minimized. Further, also in order that requiredoptical indices should be achieved, an appropriate annealing treatmentis necessary. According to this means, press molding may be performed inan ordinary atmosphere. Further, the above-mentioned mold-releasingagent may be used.

Here, in both cases regardless of whether the first means or the secondmeans described above is selected, the refractive index (nd) and theAbbe number (vd) of the obtained optical element varies slightly owingto the heat history in the producing process. Thus, when an opticalelement having precisely specified optical indices is to be produced,component adjustment for the optical glass composition, heat historyadjustment in the producing process, adjustment of incorporating theamount of variation into the optical design when necessary, or the likemay be selected appropriately with taking into consideration theabove-mentioned variation in the refractive index (nd) and the Abbenumber (vd). As a result, an optical element is obtained that hasdesired optical indices and an excellent transmissivity and that isparticularly suitable as an optical component of a device whichincorporates a solid-state image sensor or the like.

EXAMPLES

Next, the embodiments of the present invention are described below infurther detail with reference to the following examples. However, theembodiments are not limited to these examples.

Operation in the examples and comparative examples was as follows.First, raw material mixture composed of predetermined amounts of oxidesand carbonates was put into a platinum crucible. Then, the raw materialmixture was melted at 1350° C. to 1450° C. for 1 hour with stirring itintermittently. After that, the melt was caused to flow into a moldingdie having been heated in advance, then held for 1 hour in an electricfurnace set at a temperature higher than the expected Tg, and thencooled in the furnace at a cooling rate of 30° C./hour, so that anoptical glass lump was obtained. After that, using polished samples cutout from the optical glass lump, the refractive index (nd), thedispersion (vd: Abbe number) and the liquidus temperature were measuredin all of the examples and comparative examples. The compositions(component ratios) of the glass in the examples and the comparativeexamples are shown in the following tables.

In the tables, the following points are to be noted.

(1) The component ratios in the composition fields in each table areexpressed in % by mole calculated from batch materials.

(2) The “nd” and the “vd” indicate respectively the refractive index andthe Abbe number at room temperature.

(3) The liquidus temperature is obtained by a method in that thepolished samples are maintained for 1 hour in a devitrification testerwith a temperature gradient of 1000-1400° C. and then formation of acrystal on the polished samples is examined under a microscope of 40times power.

TABLE I-1 Component Example (% by mole) I-1 I-2 I-3 I-4 I-5 I-6 I-7 I-8I-9 SiO₂ 15.9 15.9 15.8 15.8 13.3 19.3 22.0 16.7 16.7 B₂O₃ 31.3 31.231.2 31.2 33.7 30.2 27.7 33.0 33.0 K₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 Na₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Li₂O 2.7 2.7 2.7 2.7 2.72.8 2.8 0.0 0.0 ZnO 11.7 11.7 11.7 11.7 11.8 6.1 6.1 13.4 13.4 MgO 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 SrO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0BaO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Al₂O₃ 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 ZrO₂ 7.7 7.7 7.7 7.7 7.8 8.1 8.0 2.7 2.7 La₂O₃ 15.3 16.217.2 18.1 15.3 17.4 17.4 16.9 17.9 Nb₂O₅ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Ta₂O₅ 2.3 2.3 2.3 2.3 2.32.4 2.4 3.4 3.4 Gd₂O₃ 13.2 12.3 11.4 10.5 13.2 13.7 13.7 13.9 12.9 Y₂O₃0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 WO₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 La₂O₃ + Gd₂O₃ + 59.859.7 59.8 59.8 62.2 61.3 58.8 63.8 63.8 B₂O₃ La₂O₃ + Gd₂O₃ 28.5 28.528.6 28.6 28.5 31.1 31.1 30.8 30.8 nd 1.84582 1.84620 1.84559 1.847621.84555 1.85066 1.85024 1.84391 1.84279 νd 44.5 44.5 44.5 44.6 44.6 44.944.3 44.6 44.1 Liquidus 1275 1275 1275 1275 1270 1275 1275 1225 1275temperature (° C.)

TABLE I-2 Component Example (% by mole) I-10 I-11 I-12 I-13 I-14 I-15I-16 I-17 I-18 SiO₂ 16.7 16.7 14.0 19.5 22.2 18.2 16.9 14.2 19.5 B₂O₃32.9 32.9 35.5 30.5 28.0 30.6 30.8 31.2 29.4 K₂O 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 Na₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 ZnO 13.4 13.3 13.4 13.3 13.3 13.4 13.5 13.6 13.4MgO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 SrO 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 BaO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Al₂O₃ 0.0 0.0 0.0 0.00.0 0.8 1.7 3.4 0.8 ZrO₂ 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.8 2.7 La₂O₃ 18.919.9 16.9 16.8 16.7 16.9 17.0 17.2 16.9 Nb₂O₅ 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Ta₂O₅ 3.4 3.4 3.43.4 3.4 3.4 3.4 3.5 3.4 Gd₂O₃ 12.0 11.1 13.9 13.8 13.8 13.9 14.0 14.113.9 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 WO₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 La₂O₃ + Gd₂O₃ +63.8 63.9 66.3 61.1 58.5 61.4 61.8 62.5 60.2 B₂O₃ La₂O₃ + Gd₂O₃ 30.931.0 30.8 30.6 30.5 30.8 31.0 31.3 30.8 nd 1.84478 1.84561 1.842481.84505 1.84526 1.84549 1.84844 1.84854 1.84503 νd 44.3 44.6 44.8 44.744.4 44.9 44.6 44.3 44.3 Liquidus 1275 1275 1270 1225 1270 1235 12351275 1235 temperature (° C.)

TABLE I-3 Component Example (% by mole) I-19 I-20 I-21 I-22 I-23 I-24I-25 I-26 I-27 SiO₂ 19.6 19.8 18.2 16.8 18.1 16.7 18.2 16.9 17.9 B₂O₃28.3 26.1 30.6 30.8 30.5 30.5 30.6 30.8 30.3 K₂O 0.0 0.0 0.9 1.8 0.0 0.00.0 0.0 0.0 Na₂O 0.0 0.0 0.0 0.0 1.3 2.7 0.0 0.0 0.0 Li₂O 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 ZnO 13.4 13.5 13.4 13.5 13.3 13.3 13.4 13.5 13.2MgO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.1 SrO 0.0 0.0 0.0 0.0 0.0 0.0 0.81.6 0.0 BaO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Al₂O₃ 1.7 3.3 0.0 0.00.0 0.0 0.0 0.0 0.0 ZrO₂ 2.7 2.8 2.7 2.7 2.7 2.7 2.7 2.7 2.7 La₂O₃ 16.917.1 16.9 17.0 16.8 16.8 16.9 17.0 16.7 Nb₂O₅ 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Ta₂O₅ 3.4 3.5 3.43.4 3.4 3.4 3.4 3.4 3.4 Gd₂O₃ 13.9 14.0 13.9 14.0 13.8 13.8 13.9 14.013.7 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 WO₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 La₂O₃ + Gd₂O₃ +59.1 57.2 61.4 61.8 61.1 61.1 61.4 61.8 60.7 B₂O₃ La₂O₃ + Gd₂O₃ 30.831.1 30.8 31.0 30.6 30.6 30.8 31.0 30.4 nd 1.84682 1.84987 1.844951.84926 1.84321 1.84160 1.84698 1.84808 1.84661 νd 44.3 44.1 44.6 44.544.5 44.2 44.7 44.5 44.2 Liquidus 1235 1280 1235 1280 1280 1280 12801295 1275 temperature (° C.)

TABLE I-4 Component Example (% by mole) I-28 I-29 I-30 I-31 I-32 I-33I-34 I-35 I-36 SiO₂ 18.2 19.4 19.5 19.4 19.4 19.6 19.2 19.6 19.6 B₂O₃30.7 30.4 30.5 30.4 30.3 30.6 30.1 30.6 30.6 K₂O 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 Na₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 ZnO 13.5 13.3 13.4 13.3 13.4 13.4 13.2 13.4 13.4MgO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 SrO 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 BaO 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Al₂O₃ 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 ZrO₂ 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 La₂O₃ 17.016.3 16.3 16.3 21.8 11.7 16.2 12.8 12.8 Nb₂O₅ 0.0 0.6 0.0 0.0 0.0 0.00.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 1.7 0.0 0.0 Ta₂O₅ 3.4 3.4 3.83.4 3.4 3.4 3.4 3.4 3.4 Gd₂O₃ 13.9 13.8 13.8 13.8 9.2 18.5 13.6 16.216.2 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.3 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 1.3 WO₃ 0.0 0.0 0.0 0.7 0.0 0.0 0.0 0.0 0.0 La₂O₃ + Gd₂O₃ +61.6 60.5 60.6 60.5 61.3 60.8 59.9 59.6 59.6 B₂O₃ La₂O₃ + Gd₂O₃ 30.930.1 30.1 30.1 31.0 30.2 29.8 29.0 29.0 nd 1.84709 1.84775 1.845121.84570 1.84592 1.84123 1.85046 1.84232 1.84045 νd 44.1 43.8 44.4 44.143.0 44.8 43.2 44.7 44.8 Liquidus 1290 1250 1250 1250 1290 1260 12601260 1250 temperature (° C.)

TABLE I-5 Component Comparative Example (% by mole) I-1 I-2 I-3 I-4 I-5SiO₂ 11.1 13.9 8.8 7.9 8.5 B₂O₃ 37.8 35.2 41.3 37.2 40.0 K₂O 0.0 0.0 0.00.0 0.0 Na₂O 0.0 0.0 0.0 0.0 0.0 Li₂O 2.8 2.8 2.8 2.5 2.7 ZnO 6.2 6.110.1 21.8 9.8 MgO 0.0 0.0 0.0 0.0 0.0 SrO 0.0 0.0 0.0 0.0 0.0 BaO 0.00.0 0.0 0.0 0.0 Al₂O₃ 0.0 0.0 0.0 0.0 0.0 ZrO₂ 8.1 8.1 6.0 5.4 10.3La₂O₃ 17.6 17.6 12.3 9.5 11.9 Nb₂O₅ 0.0 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.00.0 0.0 Ta₂O₅ 2.5 2.5 7.4 5.5 5.9 Gd₂O₃ 13.9 13.8 11.3 10.2 11.0 Y₂O₃0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.0 WO₃ 0.0 0.0 0.0 0.0 0.0La₂O₃ + Gd₂O₃ + B₂O₃ 69.3 66.6 64.9 56.9 62.9 La₂O₃ + Gd₂O₃ 31.5 31.423.6 19.7 22.9 nd 1.84884 1.85019 1.84596 1.85547 1.84999 νd 44.9 44.842.6 42.5 42.3 Liquidus 1364 1364 — — — temperature (° C.) Note: InComparative Examples I-3 to I-5, each liquidus temperature was notobtained because νd is less than 43.

As seen from Tables I-1 to I-4 given above, the optical glasscomposition according to each of Examples I-1 to I-36 has a refractiveindex (nd) to the d-line falling within a high refractive index range of1.83 or higher and 1.86 or lower and an Abbe number (vd) to the d-linefalling within a lower to middle dispersion range of 43 or higher and 46or lower, and yet has a low liquidus temperature of 1300° C. or lower.

TABLE II-1 Component Example (% by mole) II-1 II-2 II-3 II-4 II-5 II-6II-7 II-8 SiO₂ 16.7 16.7 16.7 16.7 11.2 14.0 19.5 22.2 B₂O₃ 33.0 33.032.9 32.9 38.1 35.5 30.5 28.0 K₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Na₂O0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZnO13.4 13.4 13.4 13.3 13.5 13.4 13.3 13.3 MgO 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 SrO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 Al₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ 2.7 2.7 2.7 2.7 2.7 2.72.7 2.7 La₂O₃ 16.9 17.9 18.9 19.9 17.0 16.9 16.8 16.7 Nb₂O₅ 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Ta₂O₅ 3.4 3.43.4 3.4 3.4 3.4 3.4 3.4 Gd₂O₃ 13.9 12.9 12.0 11.1 14.0 13.9 13.8 13.8Y₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 WO₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 La₂O₃/Gd₂O₃ 1.22 1.39 1.58 1.791.21 1.22 1.22 1.21 (molar ratio) nd 1.84391 1.84279 1.84478 1.845611.84092 1.84248 1.84505 1.84526 νd 44.6 44.1 44.3 44.6 45.2 44.8 44.744.4 Liquidus 1225 1275 1275 1275 1270 1270 1225 1270 temperature (° C.)

TABLE II-2 Component Example (% by mole) II-9 II-10 II-11 II-12 II-13II-14 II-15 II-16 SiO₂ 18.2 16.9 14.2 8.7 19.5 19.6 19.8 18.2 B₂O₃ 30.630.8 31.2 31.9 29.4 28.3 26.1 30.6 K₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.9Na₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 ZnO 13.4 13.5 13.6 14.0 13.4 13.4 13.5 13.4 MgO 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 SrO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 Al₂O₃ 0.8 1.7 3.4 6.9 0.8 1.7 3.3 0.0 ZrO₂ 2.7 2.7 2.8 2.82.7 2.7 2.8 2.7 La₂O₃ 16.9 17.0 17.2 17.6 16.9 16.9 17.1 16.9 Nb₂O₅ 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Ta₂O₅3.4 3.4 3.5 3.6 3.4 3.4 3.5 3.4 Gd₂O₃ 13.9 14.0 14.1 14.5 13.9 13.9 14.013.9 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 WO₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 La₂O₃/Gd₂O₃ 1.22 1.21 1.221.21 1.22 1.22 1.22 1.22 (molar ratio) nd 1.84549 1.84844 1.848541.85210 1.84503 1.84682 1.84987 1.84495 νd 44.9 44.6 44.3 43.8 44.3 44.344.1 44.6 Liquidus 1235 1235 1275 1275 1235 1235 1280 1235 temperature(° C.)

TABLE II-3 Component Example (% by mole) II-17 II-18 II-19 II-20 II-21II-22 II-23 II-24 SiO₂ 16.8 18.1 16.7 18.2 16.9 17.9 18.2 19.4 B₂O₃ 30.830.5 30.5 30.6 30.8 30.3 30.7 30.4 K₂O 1.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0Na₂O 0.0 1.3 2.7 0.0 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 ZnO 13.5 13.3 13.3 13.4 13.5 13.2 13.5 13.3 MgO 0.0 0.0 0.0 0.0 0.02.1 0.0 0.0 SrO 0.0 0.0 0.0 0.8 1.6 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 0.00.0 0.5 0.0 Al₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ 2.7 2.7 2.7 2.72.7 2.7 2.7 2.7 La₂O₃ 17.0 16.8 16.8 16.9 17.0 16.7 17.0 16.3 Nb₂O₅ 0.00.0 0.0 0.0 0.0 0.0 0.0 0.6 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Ta₂O₅3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 Gd₂O₃ 14.0 13.8 13.8 13.9 14.0 13.7 13.913.8 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 WO₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 La₂O₃/Gd₂O₃ 1.21 1.22 1.221.22 1.21 1.22 1.22 1.18 (molar ratio) nd 1.84926 1.84321 1.841601.84698 1.84808 1.84661 1.84709 1.84775 νd 44.5 44.5 44.2 44.7 44.5 44.244.1 43.8 Liquidus 1280 1280 1280 1280 1295 1275 1290 1250 temperature(° C.)

TABLE II-4 Component Example (% by mole) II-25 II-26 II-27 II-28 II-29II-30 II-31 SiO₂ 19.5 19.4 19.4 19.6 19.2 19.6 19.6 B₂O₃ 30.5 30.4 30.330.6 30.1 30.6 30.6 K₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Na₂O 0.0 0.0 0.0 0.00.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZnO 13.4 13.3 13.3 13.413.2 13.4 13.4 MgO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 SrO 0.0 0.0 0.0 0.0 0.00.0 0.0 BaO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Al₂O₃ 0.0 0.0 0.0 0.0 0.0 0.00.0 ZrO₂ 2.7 2.7 2.7 2.7 2.7 2.7 2.7 La₂O₃ 16.3 16.3 21.8 11.7 16.2 12.812.8 Nb₂O₅ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 1.7 0.0 0.0Ta₂O₅ 3.8 3.4 3.4 3.4 3.4 3.4 3.4 Gd₂O₃ 13.8 13.8 9.2 18.5 13.6 16.216.2 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 1.3 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 1.3WO₃ 0.0 0.7 0.0 0.0 0.0 0.0 0.0 La₂O₃/Gd₂O₃ 1.18 1.18 2.37 0.63 1.190.79 0.79 (molar ratio) nd 1.84512 1.84570 1.84592 1.84123 1.850461.84232 1.84045 νd 44.4 44.1 43.0 44.8 43.2 44.7 44.8 Liquidus 1250 12501290 1260 1260 1260 1250 temperature (° C.)

TABLE II-5 Component Comparative Example (% by mole) II-1 II-2 SiO₂ 10.719.3 B₂O₃ 36.1 30.2 K₂O 0.0 0.0 Na₂O 0.0 0.0 Li₂O 2.7 0.0 ZnO 11.8 13.2MgO 0.0 0.0 SrO 0.0 0.0 BaO 0.0 0.0 Al₂O₃ 0.0 0.0 ZrO₂ 7.8 2.7 La₂O₃15.4 24.3 Nb₂O₅ 0.0 0.0 TiO₂ 0.0 0.0 Ta₂O₅ 2.4 3.4 Gd₂O₃ 13.2 6.9 Y₂O₃0.0 0.0 Yb₂O₃ 0.0 0.0 WO₃ 0.0 0.0 La₂O₃/Gd₂O₃ 1.17 3.52 (molar ratio) nd1.84474 Devitrification νd 44.6 Liquidus 1319 temperature (° C.)

As seen from Tables II-1 to II-4 given above, the optical glasscomposition according to each of Examples II-1 to II-31 has a refractiveindex (nd) to the d-line falling within a high refractive index range of1.83 or higher and 1.87 or lower and an Abbe number (vd) to the d-linefalling within a lower to middle dispersion range of 43 or higher and 47or lower, and yet has a low liquidus temperature of 1300° C. or lower.

TABLE III-1 Component Example (% by mole) III-1 III-2 III-3 III-4 III-5III-6 III-7 III-8 SiO₂ 16.7 16.7 16.7 16.7 11.2 14.0 19.5 22.2 B₂O₃ 33.033.0 32.9 32.9 38.1 35.5 30.5 28.0 K₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Na₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 ZnO 13.4 13.4 13.4 13.3 13.5 13.4 13.3 13.3 MgO 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 SrO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 Al₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ 2.7 2.7 2.7 2.72.7 2.7 2.7 2.7 La₂O₃ 16.9 17.9 18.9 19.9 17.0 16.9 16.8 16.7 Nb₂O₅ 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Ta₂O₅3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 Gd₂O₃ 13.9 12.9 12.0 11.1 14.0 13.9 13.813.8 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 WO₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 SiO₂/B₂O₃ 0.51 0.51 0.510.51 0.29 0.39 0.64 0.79 (molar ratio) nd 1.84391 1.84279 1.844781.84561 1.84092 1.84248 1.84505 1.84526 νd 44.6 44.1 44.3 44.6 45.2 44.844.7 44.4 Liquidus 1225 1275 1275 1275 1270 1270 1225 1270 temperature(° C.)

TABLE III-2 Component Example (% by mole) III-9 III-10 III-11 III-12III-13 III-14 III-15 III-16 SiO₂ 18.2 16.9 14.2 8.7 19.5 19.6 19.8 18.2B₂O₃ 30.6 30.8 31.2 31.9 29.4 28.3 26.1 30.6 K₂O 0.0 0.0 0.0 0.0 0.0 0.00.0 0.9 Na₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 ZnO 13.4 13.5 13.6 14.0 13.4 13.4 13.5 13.4 MgO 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 SrO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 Al₂O₃ 0.8 1.7 3.4 6.9 0.8 1.7 3.3 0.0 ZrO₂ 2.7 2.72.8 2.8 2.7 2.7 2.8 2.7 La₂O₃ 16.9 17.0 17.2 17.6 16.9 16.9 17.1 16.9Nb₂O₅ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 Ta₂O₅ 3.4 3.4 3.5 3.6 3.4 3.4 3.5 3.4 Gd₂O₃ 13.9 14.0 14.1 14.5 13.913.9 14.0 13.9 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 WO₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 SiO₂/B₂O₃ 0.590.55 0.46 0.27 0.66 0.69 0.76 0.59 (molar ratio) nd 1.84549 1.848441.84854 1.85210 1.84503 1.84682 1.84987 1.84495 νd 44.9 44.6 44.3 43.844.3 44.3 44.1 44.6 Liquidus 1235 1235 1275 1275 1235 1235 1280 1235temperature (° C.)

TABLE III-3 Component Example (% by mole) III-17 III-18 III-19 III-20III-21 III-22 III-23 III-24 SiO₂ 16.8 18.1 16.7 18.2 16.9 17.9 18.2 19.4B₂O₃ 30.8 30.5 30.5 30.6 30.8 30.3 30.7 30.4 K₂O 1.8 0.0 0.0 0.0 0.0 0.00.0 0.0 Na₂O 0.0 1.3 2.7 0.0 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 ZnO 13.5 13.3 13.3 13.4 13.5 13.2 13.5 13.3 MgO 0.0 0.0 0.00.0 0.0 2.1 0.0 0.0 SrO 0.0 0.0 0.0 0.8 1.6 0.0 0.0 0.0 BaO 0.0 0.0 0.00.0 0.0 0.0 0.5 0.0 Al₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ 2.7 2.72.7 2.7 2.7 2.7 2.7 2.7 La₂O₃ 17.0 16.8 16.8 16.9 17.0 16.7 17.0 16.3Nb₂O₅ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.6 TiO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 Ta₂O₅ 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 Gd₂O₃ 14.0 13.8 13.8 13.9 14.013.7 13.9 13.8 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 WO₃ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 SiO₂/B₂O₃ 0.550.59 0.55 0.59 0.55 0.59 0.59 0.64 (molar ratio) nd 1.84926 1.843211.84160 1.84698 1.84808 1.84661 1.84709 1.84775 νd 44.5 44.5 44.2 44.744.5 44.2 44.1 43.8 Liquidus 1280 1280 1280 1280 1295 1275 1290 1250temperature (° C.)

TABLE III-4 Component Example (% by mole) III-25 III-26 III-27 III-28III-29 III-30 III-31 SiO₂ 19.5 19.4 19.4 19.6 19.2 19.6 19.6 B₂O₃ 30.530.4 30.3 30.6 30.1 30.6 30.6 K₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Na₂O 0.00.0 0.0 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZnO 13.4 13.313.3 13.4 13.2 13.4 13.4 MgO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 SrO 0.0 0.0 0.00.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Al₂O₃ 0.0 0.0 0.0 0.00.0 0.0 0.0 ZrO₂ 2.7 2.7 2.7 2.7 2.7 2.7 2.7 La₂O₃ 16.3 16.3 21.8 11.716.2 12.8 12.8 Nb₂O₅ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.01.7 0.0 0.0 Ta₂O₅ 3.8 3.4 3.4 3.4 3.4 3.4 3.4 Gd₂O₃ 13.8 13.8 9.2 18.513.6 16.2 16.2 Y₂O₃ 0.0 0.0 0.0 0.0 0.0 1.3 0.0 Yb₂O₃ 0.0 0.0 0.0 0.00.0 0.0 1.3 WO₃ 0.0 0.7 0.0 0.0 0.0 0.0 0.0 SiO₂/B₂O₃ 0.64 0.64 0.640.64 0.64 0.64 0.64 (molar ratio) nd 1.84512 1.84570 1.84592 1.841231.85046 1.84232 1.84045 νd 44.4 44.1 43.0 44.8 43.2 44.7 44.8 Liquidus1250 1250 1290 1260 1260 1260 1250 temperature (° C.)

TABLE III-5 Component Comparative Example (% by mole) III-1 III-2 III-3III-4 SiO₂ 8.5 8.5 24.8 40.5 B₂O₃ 40.8 40.9 25.5 10.9 K₂O 0.0 0.0 0.00.0 Na₂O 0.0 0.0 0.0 0.0 Li₂O 0.0 0.0 0.0 0.0 ZnO 13.6 13.6 13.2 12.9MgO 0.0 0.0 0.0 0.0 SrO 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 Al₂O₃ 0.00.0 0.0 0.0 ZrO₂ 2.8 2.8 2.7 2.6 La₂O₃ 14.5 11.4 16.7 16.3 Nb₂O₅ 0.0 0.00.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 Ta₂O₅ 3.5 3.5 3.4 3.3 Gd₂O₃ 16.4 19.3 13.713.4 Y₂O₃ 0.0 0.0 0.0 0.0 Yb₂O₃ 0.0 0.0 0.0 0.0 WO₃ 0.0 0.0 0.0 0.0SiO₂/B₂O₃ 0.21 0.21 0.97 3.70 (molar ratio) nd Devitri- Devitrification1.84704 Devitrification νd fication 44.5 Liquidus 1335 temperature (°C.)

As seen from Tables III-1 to III-4 given above, the optical glasscomposition according to each of Examples III-1 to III-31 has arefractive index (nd) to the d-line falling within a high refractiveindex range of 1.83 or higher and 1.87 or lower and an Abbe number (vd)to the d-line falling within a lower to middle dispersion range of 43 orhigher and 47 or lower, and yet has a low liquidus temperature of 1300°C. or lower.

TABLE IV-1 Component Example (% by mole) IV-1 IV-2 IV-3 IV-4 IV-5 IV-6IV-7 IV-8 IV-9 SiO₂ 9.0 9.1 8.5 8.5 8.5 9.1 9.1 9.1 9.1 B₂O₃ 42.5 42.743.3 43.3 43.3 43.1 43.1 43.0 43.0 GeO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 Li₂O 2.8 2.8 2.8 2.8 2.9 2.9 2.9 2.9 2.9 ZnO 4.2 4.2 4.2 4.2 4.2 4.24.2 4.2 4.2 BaO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ 8.2 7.6 7.6 7.67.6 6.3 6.3 6.3 6.2 La₂O₃ 14.4 14.5 14.5 12.5 11.5 15.7 16.7 17.8 18.8Ta₂O₅ 5.7 6.0 6.0 6.0 6.0 6.4 6.4 6.4 6.4 Ta₂O₅ + ZrO₂ 13.9 13.6 13.613.6 13.6 12.7 12.7 12.7 12.6 Gd₂O₃ 13.1 13.1 13.1 15.1 16.0 12.3 11.310.3 9.4 nd 1.84727 1.84729 1.84884 1.84686 1.84612 1.84692 1.846861.84641 1.84731 νd 43.2 44.1 43.3 43.6 43.6 44.1 43.2 43.3 43.1 Liquidus1165 1165 1175 1175 1175 1185 1145 1145 1185 temperature (° C.)

TABLE IV-2 Component Example (% by mole) IV-10 IV-11 IV-12 IV-13 IV-14IV-15 IV-16 IV-17 SiO₂ 8.7 8.6 8.0 8.5 8.4 8.4 8.9 8.6 B₂O₃ 41.1 40.540.6 43.0 43.0 42.9 41.8 40.8 GeO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 7.8 Li₂O2.7 2.7 2.7 2.8 2.8 2.8 2.8 2.7 ZnO 10.1 11.9 11.8 4.2 4.2 4.1 4.1 4.0BaO 0.0 0.0 0.0 0.0 0.0 0.0 5.4 0.0 ZrO₂ 6.0 5.9 7.1 8.2 8.2 8.2 6.1 5.9La₂O₃ 14.0 13.8 13.6 15.5 16.5 18.5 13.7 13.4 Ta₂O₅ 6.1 6.0 5.6 6.3 6.36.3 6.2 6.1 Ta₂O₅ + ZrO₂ 12.1 11.9 12.7 14.5 14.5 14.5 12.3 12.0 Gd₂O₃11.3 10.7 10.6 11.4 10.5 8.6 11.0 10.7 nd 1.84227 1.84219 1.842631.84948 1.85063 1.85045 1.83728 1.83369 νd 43.3 43.3 44.0 43.3 43.2 44.043.3 43.1 Liquidus 1155 1155 1155 1180 1140 1180 1170 1170 temperature(° C.)

TABLE IV-3 Component Comparative Example (% by mole) IV-1 IV-2 IV-3 SiO₂11.1 13.9 8.0 B₂O₃ 37.8 35.2 38.5 GeO₂ 0.0 0.0 0.0 Li₂O 2.8 2.8 2.7 ZnO6.2 6.2 11.8 BaO 0.0 0.0 0.0 ZrO₂ 8.2 8.1 7.8 La₂O₃ 17.6 17.6 15.4 Ta₂O₅2.5 2.5 2.4 Ta₂O₅ + ZrO₂ 10.7 10.6 10.2 Gd₂O₃ 13.9 13.8 13.3 nd 1.846921.85019 Devitrification νd 45.0 44.8 Liquidus 1365 1365 temperature (°C.)

As seen from Tables IV-1 to IV-2 given above, the optical glasscomposition according to each of Examples IV-1 to IV-17 has a refractiveindex (nd) to the d-line falling within a high refractive index range of1.83 or higher and 1.86 or lower and an Abbe number (vd) to the d-linefalling within a lower to middle dispersion range of 43 or higher and 45or lower, and yet has a low liquidus temperature of 1200° C. or lower.

TABLE V-1 Component Example (% by mole) V-1 V-2 V-3 V-4 V-5 V-6 V-7 V-8V-9 SiO₂ 9.1 8.5 8.5 8.5 8.5 9.1 9.1 9.1 9.1 B₂O₃ 42.7 43.3 43.3 43.343.3 43.1 43.1 43.0 43.0 GeO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Li₂O2.8 2.8 2.8 2.8 2.9 2.9 2.9 2.9 2.9 ZnO 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.24.2 BaO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ZrO₂ 7.6 7.6 7.6 7.6 7.6 6.36.3 6.3 6.2 La₂O₃ 14.5 14.5 13.5 12.5 11.5 15.7 16.7 17.8 18.8 Ta₂O₅ 6.06.0 6.0 6.0 6.0 6.4 6.4 6.4 6.4 Ta₂O₅ + ZnO 10.2 10.2 10.2 10.2 10.210.6 10.6 10.6 10.6 Gd₂O₃ 13.1 13.1 14.1 15.1 16.0 12.3 11.3 10.3 9.4 nd1.84729 1.84884 1.84815 1.84686 1.84612 1.84692 1.84686 1.84641 1.84731νd 44.1 43.3 43.1 43.6 43.6 44.1 43.2 43.3 43.1 Liquidus 1165 1175 11751175 1175 1185 1145 1145 1185 temperature (° C.)

TABLE V-2 Component Example (% by mole) V-10 V-11 V-12 V-13 V-14 V-15V-16 V-17 SiO₂ 8.7 8.6 8.0 8.5 8.4 8.4 8.9 8.6 B₂O₃ 41.1 40.5 40.6 43.043.0 42.9 41.8 40.8 GeO₂ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 7.8 Li₂O 2.7 2.72.7 2.8 2.8 2.8 2.8 2.7 ZnO 10.1 11.9 11.8 4.2 4.2 4.1 4.1 4.0 BaO 0.00.0 0.0 0.0 0.0 0.0 5.4 0.0 ZrO₂ 6.0 5.9 7.1 8.2 8.2 8.2 6.1 5.9 La₂O₃14.0 13.8 13.6 15.5 16.5 18.5 13.7 13.4 Ta₂O₅ 6.1 6.0 5.6 6.3 6.3 6.36.2 6.1 Ta₂O₅ + ZnO 16.2 17.9 17.4 10.5 10.5 10.4 10.3 10.1 Gd₂O₃ 11.310.7 10.6 11.4 10.5 8.6 11.0 10.7 nd 1.84227 1.84219 1.84263 1.849481.85063 1.85045 1.83728 1.83369 νd 43.3 43.3 44.0 43.3 43.2 44.0 43.343.1 Liquidus 1155 1155 1155 1180 1140 1180 1170 1170 temperature (° C.)

TABLE V-3 Component Comparative Example (% by mole) V-1 V-2 SiO₂ 11.113.9 B₂O₃ 37.8 35.2 GeO₂ 0.0 0.0 Li₂O 2.8 2.8 ZnO 6.2 6.2 BaO 0.0 0.0ZrO₂ 8.2 8.1 La₂O₃ 17.6 17.6 Ta₂O₅ 2.5 2.5 Ta₂O₅ + ZnO 8.7 8.7 Gd₂O₃13.9 13.8 nd 1.84692 1.85019 νd 45.0 44.8 Liquidus 1365 1365 temperature(° C.)

As seen from Tables V-1 to V-2 given above, the optical glasscomposition according to each of Examples V-1 to V-17 has a refractiveindex (nd) to the d-line falling within a high refractive index range of1.83 or higher and 1.86 or lower and an Abbe number (vd) to the d-linefalling within a lower to middle dispersion range of 43 or higher and 46or lower, and yet has a low liquidus temperature of 1200° C. or lower.

The optical glass composition of the present invention is suitable asthe material of optical elements such as lens elements contained in ashooting lens system of a digital camera. Further, the optical glasscomposition of the present invention may be applied, for example, tolens elements of a pickup optical system used in an optical head deviceor to lens elements of an illumination optical system and a projectionoptical system used in a projector device. Then, the performance ofthese devices can be improved.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodification depart from the scope of the present invention, they shouldbe construed as being included therein.

1. An optical glass composition comprising, in % by mole, 0-25.0% of SiO₂, 20.0-40.0% of B₂O₃, 3.0-15.0% of ZnO, 0-10.0% ZrO₂, 2.0-7.0% of Ta₂O₅, 6.0-25.0% of La₂O₃, 5.0-22.0% of Gd₂O₃, 66.5% or less of La₂O₃+Gd₂O₃+B₂O₃ and 26.0% or more of La₂O₃+Gd₂O₃, wherein the composition contains no Li₂O, and wherein the composition has a refractive index (nd) to the d-line of 1.83-1.86, an Abbe number (vd) to the d-line of 43-46, and a liquidus temperature of 1300° C. or lower.
 2. A preform comprising the optical glass composition as claimed in claim 1, that is softened by heating so as to be used at least for press molding.
 3. An optical element comprising the optical glass composition as claimed in claim
 1. 4. The optical glass composition as claimed in claim 1, having the refractive index (nd) to the d-line of 1.84045-1.86.
 5. An optical glass composition comprising, in % by mole, 5.0-25.0% of SiO₂, 25.0-40.0% of B₂O₃, 10.0-15.0% of ZnO, 0-5.0% of ZrO₂, 10.0-25.0% of La₂O₃, 5.0-20.0% of Gd₂O₃ and 0-5.0% of Ta₂O₅, wherein La₂O₃/Gd₂O₃ is, in molar ratio, more than 0 and less than 3.5, wherein the composition contains no Li₂O, and wherein the composition has a refractive index (nd) to the d-line of 1.83-1.87, an Abbe number (vd) to the d-line of 43-47, and a liquidus temperature of 1300° C. or lower.
 6. A preform comprising the optical glass composition as claimed in claim 5, that is softened by heating so as to be used at least for press molding.
 7. An optical element comprising the optical glass composition as claimed in claim
 5. 8. The optical glass composition as claimed in claim 5, having the refractive index (nd) to the d-line of 1.84045-1.87.
 9. An optical glass composition comprising, in % by mole, 5.0-25.0% of SiO₂, 20.0-40.0% of B₂O₃, 0-15.0% of ZnO, 0-5.0% of ZrO₂, 10.0-25.0% of La₂O₃, 0-5.0% of Ta₂O₅ and 5.0-20.0% of Gd₂O₃, wherein SiO₂/B₂O₃ is, in molar ratio, 0.25-0.90, wherein the composition contains no Li₂O, and wherein the composition has a refractive index (nd) to the d-line of 1.83-1.87, an Abbe number (vd) to the d-line of 43-47, and a liquidus temperature of 1300° C. or lower.
 10. A preform comprising the optical glass composition as claimed in claim 9, that is softened by heating so as to be used at least for press molding.
 11. An optical element comprising the optical glass composition as claimed in claim
 9. 12. The optical glass composition as claimed in claim 9, having the refractive index (nd) to the d-line of 1.84045-1.87.
 13. An optical glass composition comprising, in % by mole, 0-10.0% of SiO₂, 41.8-43.3% of B₂O₃, 0-5.0% of Li₂O, 0-12.0% ZnO, 0-10.0% of ZrO₂, 10.0-20.0% of La₂O₃, 3.0-10.0% of Ta₂O₅, 11.0-20.0% of Ta₂O₅+ZrO₂ and 5.0-20.0% of Gd₂O₃, and having a refractive index (nd) to the d-line of 1.83-1.86, an Abbe number (vd) to the d-line of 43-45, and a liquidus temperature of 1200° C. or lower.
 14. A preform comprising the optical glass composition as claimed in claim 13, that is softened by heating so as to be used at least for press molding.
 15. An optical element comprising the optical glass composition as claimed in claim
 13. 16. The optical glass composition as claimed in claim 13, having the refractive index (nd) to the d-line of 1.84612-1.86.
 17. An optical glass composition comprising, in % by mole, 0-10.0% of SiO₂, 41.8-43.3% of B₂O₃, 0-5.0% of Li₂O, 0-12.0% of ZnO, 0-10.0% of ZrO₂, 10.0-20.0% of La₂O₃, 3.0-10.0% of Ta₂O₅, 10.0-22.0% of Ta₂O₅+ZnO and 5.0-20.0% of Gd₂O₃, and having a refractive index (nd) to the d-line of 1.83-1.86, an Abbe number (vd) to the d-line of 43-46, and a liquidus temperature of 1200° C. or lower.
 18. A preform comprising the optical glass composition as claimed in claim 17, that is softened by heating so as to be used at least for press molding.
 19. An optical element comprising the optical glass composition as claimed in claim
 17. 20. The optical glass composition as claimed in claim 17, having the refractive index (nd) to the d-line of 1.84612-1.86.
 21. An optical glass composition comprising, in % by mole, 0-25.0% of SiO₂, 20.0-40.0% of B₂O₃, 0-5.0% of Li₂O, 3.0-15.0% of ZnO, 0-10.0% of ZrO₂, 2.0-7.0% of Ta₂O₅, 6-25.0% of La₂O₃, 5.0-22.0% of Gd₂O₃, 66.5% or less of La₂O₃+Gd₂O₃+B₂O₃ and 28.0% or more of La₂O₃+Gd₂O₃, and having a refractive index (nd) to the d-line of 1.83-1.86, an Abbe number (vd) to the d-line of 43-46, and a liquidus temperature of 1300° C. or lower.
 22. A preform comprising the optical glass composition as claimed in claim 21, that is softened by heating so as to be used at least for press molding.
 23. An optical element comprising the optical glass composition as claimed in claim
 21. 24. The optical glass composition as claimed in claim 21, having the refractive index (nd) to the d-line of 1.84045-1.86.
 25. An optical glass composition comprising, in % by mole, 0-10.0% of SiO₂, 30.0-45.0% of B₂O₃, 0-5.0% of Li₂O, 0-12.0% of ZnO, 0-10.0% of ZrO₂, 10.0-20.0% of La₂O₃, 3.0-10.0% of Ta₂O₅, 12.3-20.0% of Ta₂O₅+ZrO₂ and 5.0-20.0% of Gd₂O₃, and having a refractive index (nd) to the d-line of 1.83-1.86, an Abbe number (vd) to the d-line of 43-45, and a liquidus temperature of 1200° C. or lower.
 26. A preform comprising the optical glass composition as claimed in claim 25, that is softened by heating so as to be used at least for press molding.
 27. An optical element comprising the optical glass composition as claimed in claim
 25. 28. The optical glass composition as claimed in claim 25, having the refractive index (nd) to the d-line of 1.84263-1.86. 